Abstract: The 18CrNiMo7-6 steel gear shaft cracked along the axial direction during storing after machining processing. The cracking reasons of the gear shaft were analyzed by methods of macroscopic analysis, chemical composition analysis, depth measurement of carburized layer, scanning electron microscopy observation, energy analysis and so on. The results show that:there were some particles of aluminium oxides mixed into the raw material of the gear shaft due to the improper smelting process; the particles were banded during forging and drawing process, which destroyed the continuity of the material and resulted in the stress concentration; under the action of large residual stress, delayed cracking occurred to the gear shaft.