Desirable morphological traits in rice plants, such as shorter stalks and erect leaves, are being pursued in breeding programs. Rice brassinosteroid (BR)-deficient mutants display the phenotype of reduced heights and erect lamina. Therefore, biotechnology can be used for controlled deactivation of bioactive BRs in specific tissues. Here, we isolated a gene encoding for a BR-deactivating enzyme. Based on its sequence identity with a known enzyme, Arabidopsis Cytochrome P450 734A1, we identified a rice homolog named OsCYP734A6 (LOC_Os01g29150). A search of the rice T-DNA mutant population for insertional mutants of this gene revealed two alleles. These T-DNA loss-of-function mutants displayed a strongly bending lamina phenotype similar to the morphology of rice plants treated with BRs. These two mutants were named bending lamina (bla)2-1 and bla2-2. In Arabidopsis, the transcript level of a putative BR biosynthetic gene, OsDWARF4, is feed-back down-regulated in response to exogenous application of BRs. Similarly, a steady-state level of OsDWARF4 transcripts was significantly down-regulated here in the bla2 mutant background. Based on their sequence similarity, the bending-lamina phenotype in the bla2 mutant, and down-regulation of the BR biosynthetic gene OsDWARF4, we therefore propose that OsCYP734A6 (BLA2) is likely to be involved in BR deactivation.