Abstract: The microscopic structure of rice kernel is studied after sun drying in order to research the initiation and propagation theory of stress cracks of rough rice. The microscopic structure of rice kernel is observed by Scanning Electronic Microscope. The structure of the endosperm tissue of rice kernel is damaged slightly after sun drying, and stress cracks are produced. The stress cracks are also produced between the seed capsule and the endosperm tissue. The stress cracks are generated in the center of endosperm tissue first, then propagated. When stress cracks are produced, both sides of the endosperm tissue are damaged. Others are not damaged. Most stress cracks are propagated along the edge of the starch granule, and through the cell wall. The morphosis of endosperm tissue and microscopic structure of stress cracks of rice kernel are analysed. It is important through the stress cracks of rough rice to improve the observation technology. It is significant in order to estimate the quality of rice after drying.