A new methodology for evaluating the crack closure stress in fatigue crack surfaces by ultrasonics is proposed. Echo amplitudes of 50 deg. Incidence of shear wave beam against the surface connected back wall cracks in steel specimens are used. In order to account for the exact effect of closure stress on ultrasonic response, the echo amplitude is analyzed based on the artificial application of additional closure stress. The initial closure stress at no load condition is determined by the standard compliance technique. Two empirical relationships among echo amplitude, crack depth and closure stress, are established. Based on these relations, the crack closure stress and crack depth of an unknown crack are determined by inverse analysis of the measured response of the crack. The method demonstrates an excellent capability of quantitative evaluation of crack closing stress as well as crack size.