Abstract: In order to provide theory foundation for developing the insecticide with novel modes of action, the effects of 4-hexylresorcinol (4HR) on the phenoloxidase (PO) from Plutella xylostella, which is one of the key enzymes in the development process of insects, were studied. The results showed that 4-hexylresorcinol could inhibit both monophenolase and diphenolase activity of PO, partially purified by 40% saturated and Sephadex G-100 gel filtration. The median inhibitory concentrations (I50) were estimated to be 0.34 μmol/L for monophenolase and 0.50 μmol/L for diphenolase, respectively. 4HR can also prolong the lag time of PO for oxidation of L-tyrosine and was found to be a reversible competitive inhibitor with the Ki of 0.17 μmol/L. In addition, the ability of 4HR to affect copper in the enzyme directly may be an important reason to inhibit the enzyme activity.