Abstract: In order to further analysis mechanical stress crack formation mechanism of corn seeds during threshing process, provide theory support for the development of new maize seed sheller. In this paper, Hertz theory and mechanics of materials were used to analyses the critical velocity of corn seeds generated flex cracks, and the contacting process between corn seeds and threshing parts, and compared the collision process between spike tooth and plank tooth and maize seeds. It is shown that corn seeds would be producing mechanical stress cracks, even broken when the relative speed of corn seeds and threshing parts touch each other is greater than Vz; there are significant different mechanical effect between different threshing parts and different parts and corn seeds; contact area of corn seeds and threshing unit greater, the impact force received by corn seeds greater, and impact force of spike tooth greater than plank tooth.