>How could I prevent the error message "...Uniqueness Voilated..."
>while updating record into a table.
>Anyway to handle/prevent? I would like to control this error message
>with the MESSAGEBOX() function. How to detect this error? Thanks.

I guess, the source of message is some candidate or primary index. If you want to trap it, use standard 'ON ERROR do something' approach (AERROR works good here).