Gears are widely used as machine transmission parts to transfer power and change the speed and direction of rotation. The main failure modes of the gear are broken teeth and tooth surface wear, caused mainly by large residual stress. Traditional inspection methods, such as X-rays, cannot measure tooth root residual stress accurately due to the special structure of the tooth root. In this paper, an ultrasonic method involving the longitudinally critically refracted (LCR) wave was used to measure the gear tooth root residual stress. The special sensor was actuated by a piezoelectric wafer. An examination of the transmission principle of ultrasonic waves in the gear revealed that measurements of the residual stress of the gear’s tooth root using LCR waves were feasible.