Abstract:One cultivar (Teqing) and 3 hybrids (W6154s/Teqing, Weiyou 64 and Yayou Ⅱ) were used to study the changes of sucrose content in the flag leaf and grains, the seed setting percentage and grain dry weight, the grain weight in relation to the plump grain percentage, and the effect of organic nutrient on seed setting. The results indicated that seed setting percentage of hybrid rice was affected mainly by the number of imperfect grains in the middle and lower spikelet branches, especially by that in lower branches. In comparison with conventional rice, in hybrid rice, the sucrose contents in flag leaf and grains were high and the grain weight increased rapidly at early stage of grain filling, while those were lower and slowly at late stage, respectively. Hybrid rice showed much difference in seed setting percentage and grain dry weight between upper and lower branches. Low percentage of plump grains was shown in hybrid rice when the grain dry weight was the same as that in conventional rice. Plump grains of hybrid rice were more affected by the changes of organic nutrient at later stage of grain filling. The seed setting percentage and grain weight were much different in different growing season or in the case that the relationship between asource" and "sink" was changed as compared with conventional rice. The reason that the seed setting percentage of hybrid rice was lower and unstab1e was discussed.