Mechanism of inner mechanical cracks of corn seed kernel was researched by means of Cottrell piling up of dislocation theory. The formula of expanding speed and propagation route dimension of inner mechanical cracks were inferred based on theory of Griffith energy balance and fractal geometry. Crack growth rate was analyzed through fractal method. The results showed that inner mechanical cracks of corn seeds formed when part stress increased to a certain level. According to phoronomics formula of inner cracks propagation, the macro speed propagation was less than fractal propagation speed. Fractal dimension of 1.1395 was calculated by path formula of mechanical cracks propagation. The speed of cracks propagation along starch particles extending was faster than same size of starch particles.