Thank you very much Mike. Yes, you need to set "Single User" on.
I just wonder, what will happen after shrinking the database? Will the size
stays permanently? Is this the "size" that we declare the database devices
at the initial part before creating the database?

Another question, when will the "Expand" and "Shrink" of databases be required?
Thanks again.

>You're using 6.5 right? In 6.5 you use the DBCC SHRINKDB command. You'll have to set either the DBO Use Only or Single User database properties. I don't remember which. You can also shrink the database using the SEM which will set the correct database property for you.