Abstract: To solve the problem of micro-crack direction detection in metal structures, a method of measuring micro-crack direction based on harmonic nonlinearity was developed. The influence of the crack direction on the ultrasonic scattering field was analyzed by numerical simulations of the nonlinear interaction between the micro-crack and the ultrasonic wave. The simulation results show that the increase of the second harmonic and the third harmonic propagation direction of the reflected longitudinal wave is about twice that of the crack direction, and the propagation direction of the harmonic component of the transmitted longitudinal wave is almost unchanged. The above results were applied to the measurement of arbitrary micro-crack direction. The results show that the method can measure the micro-crack direction and the error is small.