Abstract: Abstract: Optimizing sowing date is the key cultivation measure to raise rice yield. Blanket-seedling mechanically transplanted rice has been used in a large production area in China. But study of how yield component characteristics of blanket-seedling mechanically transplanted rice response to different sowing date is relatively few. Therefore, there is a need for asystematic comparative study. In order to reveal the effect of sowing date on the yield component characteristics of different varieties of mechanically transplanted rice, a series of field experiments were conducted in 2012 and 2013 in the rice region of the middle of Jiangsu provinces under typical annual rice-wheat cropping rotation system with widely used rice cultivars of super japonica rice Wuyunjing 24 and indica-japonica hybrid rice Yongyou 2640. These cultivars are high in yield potential. The research were aiming to systematically analyze the dynamics of population stems and tillers, panicle characteristics, grain filling properties, and so on in different sowing date for mechanically transplanted rice. The results indicated that, with the delayed sowing date, population stems and tillers of mechanically transplanted rice increased with tillers during the earlier stage of growth, but those tillers died out rapidly during the middle and last stage of rice growth, causing a reduction of panicle maturity and obviously reducing the ratio of productive tillers to total tillers. Stem population and tillers from seedlings of early sowing dates were significantly (P<0.05) different from those in late sowing dates. When the sowing date was delayed, the panicle of mechanically transplanted rice was lessened. All of the panicle length, grain density, grain weight per panicle, ratio of No. of primary branches to No. branches of secondary branches, No. of branches per primary and secondary branch, grain number per primary and secondary branch and grain number per branch, seed-setting rate of primary branch and secondary branch were decreased. But ratio of total grains of primary branches to total grains of secondary branches was increased. Variations of characteristics in the indica-japonica hybrid large panicle rice were more drastic. There was no difference in the ratio of No. of primary branches to No. branches of secondary branches, and the ratio of total grains of primary branches to total grains of secondary branches and grains of primary branch. However, the other indexes of panicle were of significant (P<0.05) difference between early sowing and late sowing date. The postponed sowing date reduced the final weight of kernel of both the superior grains and inferior grains, resulting in grain weight decrease. With the sowing date delayed, the maximum grain-filling rate and mean grain-filling rate of superior grains were weakened. The time reaching the maximum grain-filling rate was delayed, and both the active grain-filling period and effective grain-filling time prolonged. The maximum grain-filling rate and mean grain-filling rate of inferior grains were not significantly(P>0.05) different, but maximum grain-filling rate and mean grain-filling rate in the fifth sowing date was the lowest, from which the time required to reach the maximum grain-filling rate delayed, the active grain-filling period shortened, and the effective grain-filling time prolonged. In conclusion, the suitable early sowing dates for mechanically transplanted rice would benefit rice production as followings:1) stem population and tillers of mechanically transplanted rice was steadilyincreased;2) panicles of maturity was adequate, the panicle structure was optimized, and seed setting rate was improved; and 3) the grain-filling time was longer, while the grain-filling rate was advanced and steady, especially the grain-filling characteristics of the inferior grain was optimized, more grain was filled. All the advanced characteristics would contribute to a stable and high yield. Thus, to increase rice production, sowing date of mechanically transplanted rice should be advanced to May 26 appropriately in the central part of Jiangsu province, in combination with timely transplanting and scientific cultivation and management practices.