Abstract: Failure analysis on the fatigue fracture of the rocker idler gear of a coal mining machine was carried out by means of macro inspection, magnetic particle inspection, chemical composition analysis, metallographic test and hardness test, and the cause of fatigue source micro-cracks was determined. The results show that cutting micro-cracks produced at the ring groove of the idler gear during its rework process after machining. The micro-cracks continued to expand under the alternating load of the rocker idler gear, and eventually led to the fatigue fracture of the idler gear.