>I have tried it without Multilock statement, and VFP said nothing, which worry me, if the code didn't work at all.

I don't have VFP right now, on this computer.

I am not sure if this command is scoped to the current datasession. Perhaps you already set multilocks on in the VFP preferences.

To test whether it is ON or OFF:

? set("multilocks")

* or (more appropriate for a form):

MessageBox("Multilocks is " + set("multilocks"))

>in DataEnvironment, I have all the tables set to 5 - Optimistic Table Buffering, and RLOCK() for every Replace.
>
>I want to make sure before I change the code for the whole project:
>
>I should:
>- add SET MULTILOCKS ON in the startup

Yes, but as I said, you should check whether it is scoped to the datasession.

>- delete all the RLOCK()

Correct; with buffering, there is now almost never a need to explicitly lock records.