Abstract: Annealed 40Cr steel was treated by supersonic particles bombarding (SSPB).Microhardness tester,optical microscope and transmission electron microscope were used to analyze surface microstructure of the steel.The results showed that a 60 μm deep deformed layer was formed on the steel surface,and the microhardness reached the highest of 530 HV at a certain distance to the surface.Plenty of grain boundaries were formed in the ferrite at the distance range about 50-60 μm to the surface,and the ferrite was divided into submicro grains.At the same distance range,cementite in the pearlite was curved severely but not fractured,which indicated that the cementite was able to be defomed plastically.