WordPress MU

/Base/ in config file remain default. (version 2.7)

Let say wpmu folder set to http://www.domain.com/blog, main blog admin create new post will be under http://www.domain.com/blog/blog/post

wat reason cause it has double /blog/blog for URL ?

Oops! solved.

new version must put in root directory. thats why BASE can be remain unchange. installation will auto generate /blog/ as default main blog directory.

I reinstall in root directory (public_html).
it become http://www.domain.com/wpmu/blog/post...

How to get rid of that /blog/ in URL for admin blog?

You should not get rid of /blog/ part.
You're using subdirectory install I guess.
If you remove /blog/ part imagine that you have a page domain.com/wpmu/aboutme and after that you've got a user aboutme.