Abstract: Uniform experiment design was applied to hardened 45 carbon steel with micro-plasma arc,including the design parameters such as arc current,scanning speed,gas flow and hardening distance.The structures,mechanical properties and hardening mechanism of hardened layers were analyzed by means of testing instruments,such as optical microscope,scanning electron microscope,microhardness tester and wear tester.Results show that because of the small influencing area and the extremely high cooling speed,surface hardened layer consists mainly of lath martensite and plate martensite of fine and dense structure,the microhardness of hardened layer increases from 200 HV0.2 to 600 HV0.2,and the wear resistance is about 8.5-26.7 times as much as the 45 carbon steel.