Abstract: Based on basic principle of metal magnetic memory(MMM),this paper proposes a model of magnetic flux leakage signal of gear early local micro cracks.The tangential component of magnetic flux leakage signal will peak at the crack area while the normal component will pass zero point and its gradient has the peak.The effect of the crack depth on the characteristics has also been considered.The theory is then applied to detection of micro cracks on the tooth surface of an actual gear.The microscopic analysis results demonstrate the effectiveness of the magnetic memory testing.