Some japonica rice (Oryza sativa L.) cultivars selected through ideotype‐breeding strategy with erect panicle type (EPT), compared with conventional japonica cultivars with curved panicle type (CPT), have greater spikelet density and higher yield potential. Higher yield potential of EPT cultivars, however, does not translate into actual yield because of poor grain filling. This study aimed to clarify the difference in grain‐filling patterns between EPT and CPT cultivars, and whether a low photoassimilate supply for grains or insufficient sink capability of spikelets restricts the grain filling of EPT cultivars. The results show that the percentage of grains with specific gravity ≥1.06 was lower for EPT cultivars than CPT cultivars and that the difference in 1000‐grain weight (TGW) between the primary and secondary branches of the base panicle of the EPT cultivar was greater than that of the CPT cultivar. With the severely reduced sink treatment, where the top and middle of panicles were removed, the TGW of the base primary branches of Shennong265 (an EPT cultivar) could attain the level of the top primary branches of the control, but the TGW of base secondary branches could not attain the same level. Increasing the source/sink ratio increased the speed of cell division of the base secondary branch spikelets of Shennong6, but they could not reach the level of the top primary branch grains of the control. These results suggest that base secondary panicles of EPT cultivars contribute to the poor grain filling, and a longer period and slower rate of endosperm cell division account for the poor grain filling of base secondary panicles.