Abstract: High grain broken rate is the main limiting factor encountered when mechanically harvesting maize grains. Owing to different filling degrees, the structures of superior and inferior maize grains are different. Therefore, the objectives of this study were:1) to explore the difference in mechanical threshing broken rate between superior and inferior grains and factors that influence this; 2) to select a mechanical strength evaluation index with a high correlation with grain broken rate. Two maize varieties ('Zhongyu 3' and 'Xianyu 1171') were selected, and superior and inferior grains were mechanically threshed separately. Moisture content, 100-grain weight, mechanical strength, starch grain shape, and grain broken rate were compared and analyzed for the superior and inferior grains over three mechanical threshing dates (August 9, August 16, August 23). The results showed that the broken rate of inferior grians of 'Zhongyu 3' was higher than that of superior grains threshed on August 9, August 16, and August 23; and that the broken rate of 'Xianyu 1171' inferior grains was higher than that of superior grains on threshing dates August 16 and August 23. The moisture content and 100-grain weight of superior grains were significantly higher than those of inferior grains across different mechanical threshing dates. At the same time, the superior grains had an obvious mechanical strength advantage over the inferior grains. The results also showed that there were significant negative correlations between broken rate and facade crushing strength (r=-0.46^**), broken rate and germ crushing strength (r=-0.34^*), which could be used to better reflect the breaking resistance of grains. The horny endosperm starch granules of superior grains were larger than those of inferior grains, and the floury endosperm starch granules were mainly polyhedral in the superior grains but were mainly spheroid in inferior grains. The difference in moisture content between superior and inferior grains could not reliably reflect breaking resistance. Differences in grain weight and mechanical strength were the main factors behind differences in broken rate between superior and inferior grains. The present study highlighted the importance of the differences between superior and inferior grains in maize mechanical harvesting as well as the relationship between grain mechanical strength and breaking resistance.