Abstract: According to the principle of synchronous demodulation, the modulation information in the output signal of vibro-acoustic modulation test was investigated and the nonlinear interaction of sound and vibration on the crack interface in a metallic rod was analyzed. A swept signal excitation was used to obtain a proper high frequency parameter in the test of aluminum rod samples with different size of crack, and the modulation information in the output signal was analyzed. It was found that the ultrasound wave in the structure can be separated into two parts. One is the wave which passes through the crack interface modulated by the low frequency vibration, and other wave has no any changes. The nonlinear modulation model was modified and a quantitative method for crack detection was proposed. It is demonstrated by comparing with the ultrasonic C-scanning that this method can be used to estimate the fatigue crack size in the metallic rod.