error message: user <username> does exist, using root
 

sometimes when i tries to install rpm packages, i receive such message

warning: user <user_name> does not exist - using root

what does it mean?

How are you trying to install the rpm? You always have to be root to install/remove programs :)

i normally su to root, then do

rpm -ihv <package_file_name>

then sometimes i get the warning message, i think the package is installed, just wonder what the messages mean

Strange. You are doing it correctly though ;)