Which course should I pursue after completing Diploma in mechanical engineering?

Specialization can be selected only during the Masters level programme. Also, you can pursue a short-term course in the same after completing your Diploma/ B.Tech

After completing a Diploma in Mechanical Engineering, anyone can go for short-term Certificate courses like CNC Training, CAD/CAM Training, Catia, Unigraphics, Solidworks, etc. Anyone can even go for a B.Tech degree in Mechanical engineering or a related field. Hope it helps.