40CrNiMoA gear was surface hardened by laser. Microstructure of sample was analysized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). And microhardness was studied. The fatigue resistance was tested in situ. It was shown that laser hardened layer consists of hardening layer, heat affect zone ( H AZ) and substrate after laser hardening. The microstructure of hardening layer contains fine plate mar-tensite and needle martensite. The thickness of layer is about 1. 5mm. HAZ composites martensite and ultrafine pearlite in mixture. Substrate is tempered ultrafine pearlite. Surface hardness of hardening layer was improved compared with conventional hardening. Fatigue strength of 40CrNiMo gear by laser hardening was increased by 10% compared with that of 20Cr2Ni4 carbonized.