WordPress MU

Hi...
I am using default theme and in that I want custom home page/front page .

I have created custom front page and put that file as home.html in theme directory and it works fine.

But I want that home.html 'll be call only for http://www.example.com and for other user's home page like http://www.example.com/user1 the regular theme's index.php 'll be called.

"put that file as home.html in theme directory and it works fine"
- you probably mean home.php otherwise it wouldn't work at all

"But I want that home.html 'll be call only for http://www.example.com"
Then only use that default theme for your main site.
- Copy same theme, delete the home.php and rename it for use of sub-blogs.
- Or, rename that default theme folder to something else and let the regular default theme kick in (with index.php as front page) for all other sub-blogs

Thank you....

But how can I restrict other blog user to use that particular theme to access which contains home.php

Don't use home.php. Make a custom template (sitehome.php). On the main blog, go to settings - reading - use custom page for front page. Voila.