Taking the standard material specimen of a vehicle joint and an actual auto part as study objects,the strength characteristics of materials after two typical surface strengthening processes were investigated.The results show that the material’s strength can be greatly developed after surface strengthening.However,the checking charts method which is used widely in engineering cannot accurately forecast material’s strength characteristics after surface treatment.The 40Cr material’s static strength and fatigue strength can be increased by 197% and 89.8% respectively after medium frequency induction hardening.The 20MnCr5 material’s static strength and fatigue strength can be increased by 145% and 73.5% respectively after carbonitriding hardening and S-N curve moves up,the slope becomes steeper and the turning point reduces after strengthening technology.The limited life area reduces and the unlimited life area increases.