Abstract: Methods such as chemical compositions analysis, mechanical properties test, fracture analysis, metallographic examination and superficial nitration case verification were adopted to analyze the engine crankshaft which early fatigue cracked and didn’t reach the required load accumulative total cycling times when fatigue test. The results show that the crankshaft was fatigue cracking. There were some large scale hard nonmetallic inclusions at the transition arc where the stress was rather higher, which was main reason for the early fatigue cracking of the crankshaft.The poor processing quality of crankshaft was also crucial reasons for the early fatigue cracking of the crankshaft.