Abstract: Cracks appeared on the teeth of a certain 20CrMoH gear steel after carburization treatment. The cause of gear cracking was analyzed using methods such as macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that there were dendritic manganese sulfide inclusions distributed at the grain boundaries of the material. These inclusions belonged to class II sulfides, formed in the temperature range of the second brittle zone and generated by stable eutectic reactions. The formation of manganese sulfide inclusions was related to the content of S and O elements and the cooling rate. The inclusions disrupted the continuity of the matrix, causing microcracks to initiate and continuously propagated at stress concentrations, ultimately led to cracking of 20CrMoH steel gear.