Abstract: Molecular dynamics(MD) simulations were performed to investigate the effects of grain size on the mechanical properties of nano-polycrystalline iron doped with helium(He). Simulated X-ray diffraction(XRD) was used to explore the relationship between the generation of cracks and the distortion of crystal structural in nano-polycrystalline iron during tensile deformation. The simulation results show that the peak stresses are obviously decreased due to the introduction of He atoms into the grain boundaries of nano-polycrystalline iron. In addition, it is observed that the generation and growth of intergranular cracks are significantly enhanced by He atoms which are distributed at grain boundary(GB) regions during tensile simulation. The results suggest that the intergranular cracks are promoted by GB He atoms, and the size and number of cracks increase with the increasing grain size of nano-polycrystalline iron. The separation of peak 200 and 211 are significantly observed in the XRD patterns during loading. The diffraction angle of subpeaks is higher in the XRD patterns of nano-polycrystalline iron with GB He than in that without He during the strain range from 6% to 10%, and the diffraction angle of subpeaks and the growth of cracks increase with the increasing grain size. It is demonstrated that the degradation of mechanical property caused by intergranular cracks has close relationship with the change of structural distortion obtained by simulated XRD patterns in nano-polycrystalline iron.