The Al-40Si alloy was modified by 0.5 wt% Sr addition and heat treated using the solution treatment and aging treatment. Its deforming capacity and hardness variance were investigated. The microstructures of the deformed samples were analyzed using optical microscope. The results show that the hardness increases with increasing of the deformation degree. When the deformation ratio exceeds 10.1 %, the hardness sharply increases. When the deformation ratio exceeds 20.4 %, the hardness maintains almost unchanged. In addition, although the cracks are not observed on the surface when the deformation ratio is less than 14.6 %, the primary Si phase is cracked. Thus the critical deformation ratio of the studied Al-40Si alloy is 10.1 %.