In this report an evaluating method for sliding stability of wet clutch subjected to vibratory load, where the stability is represented as dynamic response of friction force to sliding velocity, is proposed. Applying this method to the frictional vibration tester fitted with wet friction material, the situation became more unstable with the exciting frequency decreased. Then stability of the tester was examined by another method: measuring stability limit velocity under the test condition that gradient of friction force-sliding velocity curve changes continuously. Results by two methods were the same in the point that damping caused by the reason except the gradient became smaller for lower frequency, and by the fact validity of the proposed method was confirmed.