Here I am providing you with the eligibility criteria for the IIT JAM exam.
At least 55% aggregate marks (taking into account all subjects, including languages and subsidiaries, all years combined) for General/OBC category candidates and at least 50% aggregate marks (taking into account all subjects, including languages and subsidiaries, all years combined) for SC, ST and PD category candidates in the qualifying degree. For candidates with letter grades/CGPA (instead of percentage of marks), the equivalence in percentage of marks will be decided by the Admitting Institute(s). Proof of having passed the qualifying degree with the minimum educational qualification as specified by the admitting institute should be submitted by 30th September 2011
The educational qualification is different for the different courses for that you could visit the official site.