Respected Sir,I have alloted Bio-tech. in NSIT whereas I have taken admission in Thapar University(Mechanical branch).So i am confused what to do ?

Both are equally good.

Mechanical is an evergreen branch and with strong Infrastructure growth in India, there will always be a great employement potential in this field.  As for doing reserach in Mechanical, scenario is not too good since these days Research in Mechanical cannot be done without the help of Inter-disciplinary invelovement like Electronics or Chemical.

As for Bio-technology, the Field is picking up the world over and the good jobs as on date are available only at research level and with Big Corporations or Govt. org.  The path is longer since you should be prepared to study for long years eg. 4 yrs for Grad., 4 yrs for PhD and then at least 3 years for two post docs - only after studying for 11 years you can expect to be earning handsome - scenario is something like Computer Science of 1980's.

Take a decision based on your personal choices and social factors.