Polyphenol oxidase (PPO) is the enzyme responsible for enzymatic browning during the growth of insects. It is also involved in defense reactions and is related with immunities in insects. PPO,a metalloenzyme oxidase, catalyzes the oxidation of o-diphenol to o-quinone. The present paper describes the effects of benzaldehyde and its p-substituted derivatives on the activity of PPO from the fifth instar of Pieris rapae L. PPO from the fifth instar of Pieris rapae L. was purified using ammonium sulfate fractionation and chromatography on Sephadex G-100. The enzyme kinetics was characterized using L-3,4-dihydroxyphenylalanine (L-DOPA) as substrate. The results show that benzaldehyde, p-hydroxybenzaldehyde, p-chlorobenzaldehyde, and p-cyanobenzaldehyde can inhibit the PPO activity for the oxidation of L-DOPA. The inhibitor concentration leading to 50% activity lost, IC50, was estimated to be 5.90, 5.62, 2.83, and 2.91 mmol/L for the four tested inhibitors, respectively. Kinetic analyses show that the inhibitory effects of these compounds are reversible. Benzaldehyde, p-hydroxybenzaldehyde, and p-chlorobenzaldehyde are noncompetitive inhibitors while p-cyanobenzaldehyde is a mixed-type inhibitor. The inhibition constants were determined for all four inhibitors.p-chlorobenzaldehyde and p-cyanobenzaldehyde were more potent inhibitors than the other compounds. These results provide a basis for developing PPO inhibition-based pesticides.