Mr. BH, have you gotten the one year multiple entry F visa in South Korea? Some friends told me that it is difficult to get a multiple entry visa in a third country, they all advice me to get a Chinese visa in USA, however, I stay in China now and want to apply for a new multiple entry Chinese visa in Seoul.

I was not able to get a multiple entry visa, all I could get was a double entry 30 day L visa. The agent who helped me in fact said that he'd never even seen a visa before with 120 days (or longer).
Seems almost impossible to get anything with more than 30 days in Korea if you're not Korean.
I even had an official stamped invitation letter from a Chinese company which I used before in NY without a problem but it was of no use here. The agent told me that the Chinese Embassy in Korea only accepts invitation letters issued by the Chinese govt.

Mr. BH, thank you very much for the information, I will ask my wife to apply for multiple entry visa valid for one year in Los Angeles for me, the fee is same for all types of visa, 130$ for single entry visa, double entry visa or multiple entry visa. I like to get a multiple entry one.

Glad I could help. One other word of advice though, I am not sure if it is possible to apply for a visa in the US if you are not in the US - that is your passport won't have the stamp indicated you've left China and entered the US. Might want to look into that in advance.

Mr. BH, thank you very much! I will look into this issue and would like to have a try to do so. If everything goes well, I will get a Chinese visa in this way.