When GA4 was applied to heading stage, it was examined to understand the change of plant hormones and starch during grain filling and ripening. Exogenous gibberellin caused a dramatic decrease in endogenous ABA content. Endogenous GA4 content in both superior and inferior part was more promoted in GA4-treated rice grain than in the control. GA1 content of an inferior part was not detected in the control and GA4-treated rice otherwise GA4 was detected in all grain parts. Ripened grain rate in GA4-treated rice grain was lower than that of the control plant. Amylopectin from GA4-treated grain contained more very short chains with degree of polymerization (DP) between 4 and 8 than amylopectin from the control plant. It suggests strongly that fine structure of rice endosperm may be changed by exogenously applied GA4 in rice plants.