Trying to remember the title of a sci-fi movie where a man finds an alien weapon in a desert that whenever he placed it in his hands, he turns to a 'monster'. I also remembers that the aliens tries to look for this lost weapon.

Sounds like the "Guyver"

Nice try but I think it's not 'guyver'. Additionally, I remember the arm weapon was lost in desert and was found by a man which turns him into a monster whenever he place it in his arm. I recall that man died/killed by the aliens in the ending. This is a late 70's or 80's movie

:) I looked long and hard... Lazerblast!

Wow , great, this is it!!
Thanks a lot