There are lots of useful infromation in this forum. Now and then, I want to use some discussion as reference in my report or paper. But, how can I cite that? Should like unpublised work or private communication? What's the format? What's your suggestion on that?

The intention of this forum isn't really to be a permanent storage which you can reference. But if you want to do that the best way to rerence something here is probably to give the main URL, which Forum it is, Message Title, Author and Date. Something like this:

"How to cite reference From this forum?" by Sheng Tue, 9 May 2000, 5:40 a.m., CFD Online Discussion Forums, Main Forum, URL: http://www.cfd-online.com/Forum/

You could also give message number and direct URL but that is less certain to be permanent - messages are moved regularly from the Main forum to the archive. With the information above it should be possible to track down this message for a long time in the future.