The flowering duration of spikelets within a panicle of the rice cultivars of Wuyujing 3 (Japonica), Yangdao 4 (Indica), Shanyou 63 (Indica hybrid rice) and PC311/Zaoxiandang 18 (Indica/japonica hybrid rice) was 5d, 7d, 7d and 8d, respectively. The spikelets flowered on the 2nd day produced the highest endosperm weight (EW) and the most endosperm cells (ECs), and followed by the spikelets flowered on the 1st day. ECs decreased with the delay of flowering of the spikelets flowered from the 3rd day. Within a variety or a hybrid combination, the difference in endosperm cell weight (ECW) was not significant among the grains flowered on different dates. EW and grain-filled percentage (G-FP) were very significantly correlated with ECs, but not with ECW. The earlier the spikelets flowered (except those flowered on the 1st day), the greater the initial proliferation power (R°), the higher the maximum proliferation rate (Vmax) and the higher the mean proliferation rate (V) of ECs, and the shorter the time reaching Vmax, and vice versa. R°, Vmax,V and ECs were significantly correlated with the physiological activities (ATPase activity and the content of spermidine and spermine) of the grains at early grain filling stage. The physiological activities of the grains, R°,Vmax,V, ECs and EW significantly increased after removing 1/2 plants at booting stage and spraying 6-BA 〔N6(benzyl) adenine〕 at early heading stage, and the results were reversed after cutting 1/2 leaves at booting stage. These results suggest that the difference in ECs results in the difference in the grain weight, while the low physiological activities of late-flowered grains are the important reasons for their poor endosperm development.