It's not mandatory per se, but a PhD is essentially research training, so one normally does it to acquire the necessary skills. Many research positions would also be precluded for someone without a PhD.
Mr Zahid, it's not mandatory to have a PhD or M.Phil for doing research. I have guided students from Masters and Bachelors too. For conducting any research you need to have a Supervisor who will guide you to go in the right direction. So if you want to do research please select some good supervisor.
Of course, after PhD, the vision of the individual broadens and he can understand the subject better. Thats why PhDs are prefered over others for research.