Abstract: The sun gear of a heavy-duty high speed gearbox cracked during assembly. The reasons for the cracking of the sun gear were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the internal inclusions caused the sun gear initiate cracks after maching and before assembly, and the machining residual stress caused the sun gear to low stress brittle cracking.