Abstract: To screen the obvious inhibitors of phenoloxidase (PO) from Pieris rapae (L.) and search after a theoretic foundation for developing the insecticide with novel modes of action, the inhibitory effects of 2-hydroxy-4-methoxybenzylidene-aniline as an inhibitor on PO from P. rapae, which had been partially purified, was studied by using microtitration assay. The results showed that the tested compound could inhibit PO activity obviously and the inhibitor concentration leading to 50% activity loss (IC₅₀) was estimated to be 0.116 mmol/L. The inhibitor was found to be a reversible noncompetitive inhibitor, and the K_i value was determined to be 1.96 mmol/L for the compound. Although the compound could affect PO directly, it did not chelate copper of PO active center.