Rice bran was extracted with water and its effects on potato and banana polyphenol oxidase (PPO) were investigated. Rice bran extract (RBE), conc. 0.3 g mL−1, exhibited PPO inhibition in potato and banana PPO with % inhibition of 69.31% and 47.63%, respectively (P ≤ 0.05). RBE showed a concentration-dependent inhibition on potato and banana PPO. RBE (conc. 0.3 g mL−1) inhibited potato PPO higher than ascorbic acid, citric acid, NaCl and EDTA (final conc. 20 mg L−1); and it also inhibited banana PPO higher than citric acid, NaCl and EDTA (final conc. 20 mg L−1), respectively. The combination of RBE with citric acid or ascorbic acid appeared to be additive inhibitory effect on banana and potato PPO. Kinetic study of the inhibition on potato and banana PPO by RBE showed that RBE was a mixed-type inhibitor; however, RBE appeared to be able to act directly on enzyme structure rather than substrate structure.