Abstract: The three-dimensional photoelasticity method is used to analyze the stress distribution in the bevel gear under torsional moment and centrifugal load. The results show that the stress distributions in roots of the bevel gear are different under respective loads. According the results and the comprehensive vibration test data, it can be concluded as follows: 1. The cracks at the nothes of the bevel gear teeth are not caused by starting torque. 2. At high gearing speed the combined stresses of centrifugal load and resonance vibration at the notches are the main cause for the catastrophic failure of the bevel gear.