To the problem of 20CrMnMoA gear cracked along the radial direction during the test running, the microstructure were analyzed by magnetic particle examination, fracture examination, scanning electron microscopy and optical microscopy examination. The results show that there are many cracks on the fracture surface, sulfide inclusion in cracks, and partial cracks extended along sulfide inclusion. The cracks in carburized layer and the microstructure near the cracks is normal. The microstructure of gear meets the relevant technical standards.