The effects of grain boundary on the deformation behavior of magnesium bicrystals under tensile loading are investigated using molecular dynamics simulation method. Deformation mechanisms of both asymmetric and symmetric magnesium bicrystals are studied. The results show that the twist angle could exert a significant influence on the flow stress in the plastic stage. It is found that the plastic property of symmetric magnesium bicrystal is much better than that of asymmetric counterpart. In addition, for bicrystalline metal with large twist angle, its plastic response is better than that with small angle, owing to the dislocation nucleation activity in the grain boundary region.