Abstract: Aiming at the detection of micro crack in structures,a non-collinear mixed frequency ultrasonic method for detection of closed cracks was developed.Based on theoretical analysis,non-collinear mixed frequency ultrasonic testing experiment was conducted on closed cracks in structure.Testing signals in both intact and cracked specimens were filtered and analyzed by time-frequency method.Results showed that the closed crack can be recognized by the emergence of obvious wave form in time-domain filtered signal,or the sum-frequency component in frequency domain.By changing locations of excitation probes,the confluence position of two incident waves can be controlled and non-collinear mixed frequency ultrasonic testing at different depths was achieved.Moreover,according to the distribution of mixed frequency nonlinear parameter the length of closed crack along depth was measured.The research makes some useful efforts on quantitative evaluation of micro-crack.