This study shows a method of detecting the existence and size of a small crack in a root between two crests in threads using ultrasound. The key idea is from the fact that the Rayleigh wave propagates slowly along a crack from the tip to the opening and is reflected from the opening mouth. When there exists a crack, a small delayed pulse due to the Rayleigh wave is detected after a large regularly spaced pulse from the thread. The delay time is the same as the propagation delay time of the slow Rayleigh wave and is proportional to the size of the crack. The accurate detection methods of the existence of the Rayleigh wave pulse and the delay time are presented. These methods include the predictive deconvolution which effectively eliminates the regularly-spaced thread echo pulses. Theoretical explanation is presented and the experimental results of detecting cracks as small as 0.5 mm are also shown.