I want each site in sharepoint 2010 should have different scope list in dropdown. currently each site show same scope in dropdown list.
pls help me to do this or suggest any solution for this
Out of the box, you define search scopes per site collection. If you want sub sites to have their own search scope settings, you will need to create them as site collections (SPSite) not sub webs (SPWeb)
One way to acheive this would be to use the techniue outlined in the following blog post to replace the default site search box added via the delegate control with the actual SearchBoxEx web part which you are then free to customise per site.
Once you have added the web part, create a display group for each site in the site collection search scopes and then you are free to modify the property of the web part to show a custom per site.