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Mechanical threshing used when preparing maize seeds for planting subjects seed to damage and reduces seed quality. The purpose of this study was to assess the effect of mechanical threshing on the quality of maize seed threshed at different moisture contents (MCs). Seeds of dent maize JK968 and flint maize DD2 were threshed at 12, 15, 18, 21, and 24% MC. The damage degree was determined by iodine staining, and seed vigor was assessed by standard germination (SG), cold test germination (CTG), accelerated aging germination (AAG), seedling emergence rate, and seedling root and shoot length. The results showed that the damage percentage increased, and the seed vigor parameters decreased with increasing seed MC during threshing in both cultivars. For obtaining high seed quality, indicated by at least 90, 85, and 80% of SG, AAG, and CTG, respectively, JK 968 and DD2 should be threshed at MC lower than 15 and 18%, respectively. Furthermore, the damage mainly occurred in the apical part of seeds, irrespective of the threshing MC in both cultivars. When the embryo was damaged, seedling emergence rates were significantly reduced with great influence on shoot length. Damage to the endosperm resulted in little effect on seedling performance. Flint maize DD2 was more tolerant to mechanical threshing than dent maize JK968. These results provided technical reference for the production and processing of high vigor maize seeds.