Abstract: Some gears occurred cracking after carburizing,quenching and gear grinding. The cracking reason was analyzed by means of fracture analysis,chemical composition analysis,macrostructure test,hardness test,metallographic test and so on. The results showed that the carbonization level was so high after carburizing,there were lots of centralized stress on the teeth of gear after quenching,and the grinding cracks appeared when the teeth were grinded,finally due to crack during deposited in the depository.