In order to analyze the key factor to affect fatigue life of drill tools, fracture morphology and microstructure of defect samples made of 23CrNi3Mo with different types was observed by optical microscope and scanning electron microscope(SEM), and micro-hardness was tested, then failure reason was analyzed. The results show that the plastic deformation features of defect sample with high fatigue life is obvious during fatigue cracks generation and expanding, which has good ability to resist fatigue crack propagation, while impacter drill defect sample appears brittle fracture features. The surface of Atlas and impacter drill both have carburized layer with high hardness. The wide transitive layer of Atlas drill renders the strength of carburized layer and the roughness of matrix well matched and has well hardness gradient, while the impacter doesn’t. Thus the hardness gradient and the matching of strength and toughness affect the drill fatigue life heavily, and carburization and heat treatment process are very important.