If you look further into the startDrag function in the Help documentation you will see that it includes parameters that you can specify to limit the areas in which in can be dragged.
startDrag(target, lock, left, top, right, bottom)
If you intend no movement in the _y direction, then you would specify the same values for the top and bottom parameters
I already read the documentation about the startDrag function but it did'nt explain how to get the specific left, top, right and bottom value of the a movieclip that makes the draggable area.
I try to change the said parameters in startDrag function but I did'nt get the exact values to make my square movieclip draggable only on the area of the rectangle movieclip.
I know I'm still around the bushes but somehow I know I'm on the right direction.
Thanks for the reply guys,