A rice dominant gene, Ur1, increases spikelet number per panicle, thereby enlarging sink size. The effects of Ur1 on panicle and grain traits were examined using 'Nishihikari' (N), its Ur1 isogenic line (NU) and their F1, and the sd1-d isogenic line of Taichung 65 (d), the sd1-d-Ur1 isogenic line (u) and their F1. Ur1 increased number of primary branches per panicle (NB1), number of secondary branches per primary branch (NB2) and number of spikelets per single secondary branch (SB2). Increase in NB1 was higher in NU than in u but those in NB2 and SB2 were lower in NU than in u, all of which brought about a lower percentage of secondary-branch spikelets for NU. Regarding secondary-branch spikelets as well as whole spikelets, NU had a higher ripened-grain percentage caused by its higher fertilized-spikelet percentage than u. The above characteristics of NU contribute to its high yielding ability, suggesting that N is a favorable genetic background for Ur1. In addition, number of differentiated (developed+degenerated) secondary branches per primary branch and presence of twined spikelets in the uppermost primary branch of a panicle could be new indicators to discriminate Ur1/Ur1 plants from Ur1/+ and +/+ plants in a segregating population like an F2.