Will U.S Embassy give a B1/B2 visa to a dual citizen even one of his nationality is USA?

No. See 22 CFR 40.2(a):

(a) Nationals of the United States. A national of the United States shall not be issued a visa or other documentation as an alien for entry into the United States.

(Note: I have read in some third party sources about certain US consulates issuing "pro-forma visas" to US dual-nationals who would otherwise be unable to leave the foreign country; but I can't seem to find any official information on this, and don't know whether it's a "visa" or just something that looks like a visa but isn't a visa.)

As with every conceivable issue regarding U.S. Passport and Visa regulations, this is covered extensively in the Foreign Affairs Manual.

First off, it depends on how old this traveller is, and who they are travelling with. For instance, 7 FAM 085 U.S. PASSPORTS AND VISAS AND DUAL NATIONALITY states:

a. Section 215(b) of the INA (8 U.S.C. 1185(b) and 22 CFR 53 require U.S. citizens to enter and depart the United States on U.S. passports, with limited exceptions.

d. See 9 FAM 40.2 Notes, 9 FAM 42.12 N4 (2)(b) and 9 FAM 42.71 N5.

What are these "limited exceptions", you might ask? 9 FAM 202.1-2 has you covered.

Applications for Visas for Certain Dual National Children:

(1) You should advise parents who apply for visas for dual national children that regulations prohibit the issuance of a visa or other documentation to a U.S. citizen or national for entry into the United States as an alien. The children of foreign government officials, however, may use their foreign passport for entry into the United States.

(2) After the U.S. citizenship of a child has been determined by a citizenship officer, the consular officer may, to avoid delay or difficulty, give a written statement to the parents for presentation to carriers or immigration officials. The statement should make clear that the bearer of the foreign passport is a dual national child of a foreign government official or employee who is traveling to the United States on official business and as such may enter the United States on the foreign passport as an exception to the provisions of INA 215(b) regarding valid passport requirement.

(3) A child under 12 years of age who is included in the passport of an alien parent in an official capacity may be admitted if evidence of U.S. citizenship is presented at the time of entry. A determination of the child’s citizenship should be made by citizenship officer prior to departure from a foreign country and the parent should be instructed to have evidence of such citizenship available for inspection by the admitting Department of Homeland Security Officer.

So, if your friend is travelling with his parents, and they're representatives on an official assignment, he can ask a consular officer for a letter stating such.

If, however, he just wants to avoid being identified as a U.S. citizen as he leaves his country, he is out of luck. It is illegal to issue U.S. visas to U.S. citizens.

The answer may be found in 8 USC § 1101:

(a)(3) The term “alien” means any person not a citizen or national of the United States.

(a)(26) The term “nonimmigrant visa” means a visa properly issued to an alien as an eligible nonimmigrant by a competent officer as provided in this chapter.

So, the US does not issue visas to its citizens.

The large majority of countries do not check whether departing travelers have the documents they need to enter the next country on their itinerary. If your "Country A" does do such checks, there are two possibilities:

1. The country permits dual citizenship. In this case, show your US passport to demonstrate your eligibility to enter the US.

2. The country forbids dual citizenship. In this case, you can either give up citizenship of one of the two countries, or you can travel through a third country. For example, if you are a citizen of the US and China, as a comment suggests, you could get a visa for the Japan in your Chinese passport, and book two separate flights: one from China to Japan, and another from Japan to the United States.

Thank you everyone and some answers are helpful. Here are the right solution I just figured out after visited Exit-Entry Administration of this Country A at hours ago (yes it is China...)

So first, it is very clear that US embassy will not issue a visa to a dual citizen that one is USA, including minor.

Secondly, transfer flight via a 3rd country is a solution, but may need extra work on time management and flight booking.

Third is the easiest way. This is a passbook called "One-time Immigration Permit", which is issued by this Exit-Entry Administration, for the people who has "Nationality Conflicts" (What an interesting term to describe such person). In most of cases, this passbook is for helping "Nationality Conflicts" minor to travel aboard, allow to have one exit and one entry at Country's border (Airport), the travel period is limited within 3 months.

Generally speaking, the supporting document for this permit passbook is super easy to collect by comparing other unbelievable permit in this country. So what you need to do is just go to the administration office to get the most updated requirements, take about 1 week to apply, and only cost US$ 3.00.

To sum up, even the country does not permit dual-citizenship, but it still provides a way to help the needed personal (Minor and partners) to travel aboard. It is cheap and quick.

Thank you again.

Every citizen of the United States must enter (and exit) the United States on her/his United States passport.

So such a visa would be useless (well, almost - see user102008's comment below). While it is probably not spelled out in the rules & regulations, a visa whose usage violates US laws is most likely refused.