Gibberellin (GA) 3β-hydroxylase catalyzes the final step of active GA synthesis in plants. In rice, the enzyme is encoded by at least two genes (OsGA3ox1 and OsGA3ox2) with differential patterns of expression. Previously, we reported that the OsGA3ox2 gene is identical with the D18 gene. The loss-of-function mutants of D18 designated as d18-AD (Akibare waisei), d18-Id18h (housetsu waisei), and d18-dy (Waito-C) show severe dwarfism caused by the decrease of the level of active GA, GA1. The gene encodes GA 3β-hydroxylase that catalyzes the reaction from GA20 to GA1 and its expression is also regulated by active GA through a feedback mechanism, suggesting that the step catalyzed by the enzyme may be a key step for regulating the level of active GA. To determine the possibility of reducing the height of rice through regulation of the level of active GA, we produced transgenic rice plants expressing the antisense of D18 cDNA. Plants expressing the antisense showed a semi-dwarf phenotype relative to the control plants carrying the vector only. The antisense plants with the semi-dwarf phenotype showed a reduced level of D18 mRNA. In contrast to the reduced level of D18 mRNA, the plants induced a higher expression level of OsGA20ox1, which encodes GA 20-oxidase catalyzing the step from GA53 to GA20 in the GA biosynthetic pathway. These results demonstrate that the height of rice can be controlled by manipulation of the GA 3β-hydroxylase expression through the decrease in the active GA level.