In this paper, MgB2 samples were sintered in flowing welding grade Ar. The effect of the sintering time varying from 10 minutes to 3 hours on superconductivity of MgB2 has been investigated. It has been found that both Hc2 and Hirr increase with the sintering time and they have linear relationships with microstrain. The sample sintered for 30 min exhibits the highest Jc at high fields. The reason can be attributed to the improved connectivity and the increased Hc2.