Abstract: Manufacturing defects such as missing pellet surface（MPS） defect are inevitable in the manufacturing of pellets, which may result in over local stress on the cladding and cause cladding failure. In this paper, under the calculation frame of ABAQUS software, the related irradiation effects and thermal effects of fuel rods were introduced into the simulation based on vairous kinds of user defined subroutines, and then the thermal –mechanical behaviors of nuclear fuel rods were simulated. The sensitivity analysis of MPS defect sizes effect on thermal –mechanical behaviors of fuel rods were completed. The simulation results found that MPS defect causes higher center temperature, and the tensile stress and comprehensive stress appears on the cladding surface coinstantaneous. These effects would be more obvious with the increasing of MPS size. The defects with relatively bigger sizes should be concerned because these defects may threat the integrity of nuclear fuel rods during the operation period.