By means of microstructure observation, microhardness, tensile and rotating bending fatigue tests, the effects of austemperingtreatment on the formation of lower bainite, carbide precipitation and mechanical properties of M50NiL bearing steel werestudied. Results showed that multi-layer composite microstructure was formed in the steel after austempering treatment. Themicrostructure of the surface layer was mainly consisted of lower bainite, martensite and carbides, that of the sub-surfacelayer contained granular carbides, a little lower bainite, lath martensite, twin martensite and thin film retained austenite, andthat of the core region mostly contained lath martensite. In contrast with the traditional martensite quenching treatment,there was more lower bainite formed in the surface layer after austempering treatment. The appearance of acicular bainitedramatically decreased the effective grain size by refining the microstructure, and there were more finer carbides precipitatedin the lath martensite. After austempering treatment and tempering, the average hardness in surface layer of the steel increasedfrom 746.5 HV0.5to 786.4 HV0.5,and the rotating bending fatigue limit of the steel increased from 870.9 to 1170.0 MPa, butthe tensile strength of the steel decreased slightly. The effects of austempering treatment on the mechanical properties hada close relationship with the carbide precipitation behavior and the formation of lower bainite.