The rotating bending fatigue crack sources of SAE52100 high carbon bearing steel were studied in this paper. On the fatigue fracture surfaces, inclusions for crack initiation are mainly Ca-Al-O and TiN inclusions, and the size of TiN inclusions is much smaller than that of Ca-Al-O inclusions. The analysis of inclusions by Aspex shows that the number and size of Ca-Al-O inclusions in the matrix are much larger than those of TiN inclusions. Combined with the calculation of stress intensity factor and the analysis of stress concentration factor, the sharp angle characteristic of TiN inclusions is the main reason for its stronger deterioration to fatigue properties, and the damaging effect of TiN inclusions with the same size to the matrix fatigue properties is about 1.33–1.67 times that of Ca-Al-O inclusions.