Abstract: The evolution of helium clusters during the annealing and tensile processing on the different tilt grain boundaries (GBs) at 300 K in α-Fe was studied applying atomistic simulation of Fe-He potentials developed by Gao, et al., and the effect of the void defect to the helium clusters and the strength during the tensile were analyzed. The results show that the helium atoms are easier to gather to form helium clusters in the void on the grain boundary during the annealing and the micro cracks occur at the intersection of void and grain boundary and the tangent position between void in the bulk and grain boundary which are the fracture initiation points. Comparing the stress-strain curves, it is shown that the fracture stress and strain of the twin grain models in which the void is located in the bulk of α-Fe are smallest.