Abstract: The effect of laser transformation hardening on the surface property and microstructure of mild steel 10CrNiMo under different laser power and cooling conditions was studied.A hardened layer on the surface of the mild steel was obtained whose microhardness is 100HV～150HV higher than that of the original mild steel The microstructure of laser processed zone was observed with a metallographic microscope and the hardness of laser processed layer was measured with a microhardness tester.The results showed that the quench zone consisted of martensite,retained sorbite,troostite,tempered sorbite and that the microstrcture in the lap zone was martensite and some sorbites locating in the middle of martensite.Because of the lower carbon content of steel 10CrNiMo and different carbon diffusion coefficient,the highest hardness did not appear on the surface but in the subsurface.In the softened zone,because the martensite produced by the first laser scanning was tempered by the second laser beam,the carbon in the martensite separates and changes into temper sorbite.Microhardness of lap zone declined due to the temper of second laser scanning.Laser transformation hardening can improve the surface hardness 100HV～150HV higher and keep good tenacity.Furthermore,laser cladding can be taken to increase the hardness more evidently.