Starch is glucose polymer linked by α-1,4 and α-1,6 glucosidic bonds. The balance of activity between starch synthases (SSs) and branching enzymes (BEs) is important for formation of amylopectin structure. SSI and SSIIIa isozymes account for 60 and 30%, respectively, of soluble SS activity in developing rice endosperm. Complete loss of both SSI and SSIIIa activities induces sterility. By contrast, a double mutant (ss1^L/ss3a) generated by crossing the leaky ss1 mutant, whose SS activity is approximately ca. 16% of the wild type, and the ss3a null mutant is fertile. Although there is only a significant residual SS activity in the developing endosperm of ss1^L/ss3a, the yield and growth of the double-mutant line is sufficient. We analyzed the amylopectin chain-length distribution, thermal and pasting properties, morphologies of starch granules and amyloplasts, and crystallinity of endosperm starch in ss1^L/ss3a and the parental mutant lines. The chain-length distribution pattern of ss1^L/ss3a was similar to that of ss3a and there was an additive effect of the reduction of SSI activity on the chain-length of amylopectin in the ss3a background. These changes in the amylopectin chain-length distribution were considered to increase the gelatinization temperature of the starch. The gelatinized starch viscosity of the double-mutant and ss3a lines was very low. Most ss1^L/ss3a starch granules were round-shaped, and the starch granular size was smaller than those of the parental mutant lines and the wild type. The starch crystallinity of ss1^L/ss3a was slightly reduced compared with that of the wild type.