Abstract: The mechanic characteristics of 6 varieties with different lodging-resistance at 3 nitrogen levels and 3 planting densities were determined．The results showed that in the same density, as the amount of nitrogen increased, bending stress, plant height, the basal internode moment are gradually increasing; but the Lodging index reduced at first, and then increased, that is, in middling fertility area, the smallest lodging index ,the least possibility of lodging. At the same level of nitrogen, as the planting density decreased, the basal internode moment, secontion modulus and lodging index increased after the first reduced, bending stress increased and then decreased, while breaking moment has increased significantly. That is, in the appropriate level of fertilization, planting should not be too thin nor too dense. In high fertility area planted thinly and in low fertility area planted densely, which may reduce the lodging index. Sparse conditions, the changes of the basal internode moment only depend on the number of stem materials to be achieved; in dense-planting conditions, the changes of the basal internode breaking moment are through the changes of section modulus and bending stress two factors together to be acheieved. In the actual production, the amount of nitrogen can neither too high nor too low, further more, different types of species, require different volume of nitrogen and planting density. The improvement of the quality stem of existing high-yield varieties may be an effective means of further improved lodging -resistance of rice.