Abstract: Aiming at the ultrasonic detection problem caused by unfavorable orientation of radial-axial crack in aero-engine disk and rings, the specimens with fatigue cracks of 1.5-10 mm in length were produced by mechanical loading method. The detection capability of four methods for radial-axial cracks was compared, including defect echo methods and back-wall echo monitoring methods with normal incidence and small angle oblique incidence longitudinal wave, respectively. The influence of probe parameters and crack location on the detection capability was analyzed. Finally, the optimal detection method was verified on a turbine disk. The research reveals that, the back-wall echo monitoring method with a small angle oblique incidence longitudinal wave has the highest detection capability for radial-axial cracks. All fatigue cracks with length of 1.5-10 mm can be detected by using a 10 MHz focusing probe with focal length of 89 mm and oblique incidence angle of 2°. The method is more applicable for cracks that near the focus point.