Abstract: The fatigue fracture and crack propagation of XCQ16 and 20Mn2 axle steel were observed by SEM. Based on the observed results, the mechanisms of fatigue crack initiation and propagation were studied. It is shown that cracks initiate from the sample surfaces. The main mechanism of crack initiation is non-metallic inclusions and dislocations. The crack closure effect of XCQ16 steel is stronger than 20Mn2 steel in crack propagation areas, leading to a lower crack growth rate. The fracture mechanisms of the two steels are different:plastic fracture for XCQ16 and cleavage fracture for 20Mn2. Non-metallic inclusions in XCQ16 are composed of oxide inclusions and sulfide inclusions, but mainly oxide inclusions in 20Mn2 steel. The mixed inclusions in XCQ16 steel have less effect on its fatigue property.