GA20-oxidase (GA20-ox) is a key enzyme involved in the biosynthesis of gibberellic acid (GA). To investigate its role in plant growth and development, we suppressed MdGA20-ox gene expression in apple (Malus domestica cv. Hanfu) plants by RNA interference (RNAi). After 20 weeks of growth in the greenhouse, significant phenotype differences were observed between transgenic lines and the nontransgenic control. Suppression of MdGA20-ox gene expression resulted in lower plant height, shorter internode length, and higher number of nodes compared with the nontransgenic control. The expression of MdGA20-ox in transgenic plants was significantly suppressed, and the active GA content in transgenic lines was lower than that in the nontransgenic control. These results demonstrated that the MdGA20-ox gene plays an important role in vegetative growth, and therefore it is possible to develop dwarfed or compact scion apple cultivars by MdGA20-ox gene silencing.