Abstract: The gear teeth of 4340 steel fractured after tested for about 4 hours on the testbed. The causes of the broken teeth were analyzed by macroscopic observation, dye penetrant test, low magnification test, grinding burn test, metallographic examination, chemical composition analysis, non-metallic inclusion evaluation, average grain size evaluation, hardness test, nitriding layer depth test, scanning electron and energy spectrum analysis. The results show that quenching cracks which had existed before the operation of the gear propagated due to meshing force during the operation until fracture.