Abstract: Phenoloxidase (PO) is one of the key enzymes in the insect molting process. Its inhibitors might ultimately provide clues to control insect pests. Based on the theory of "insect development and reproduction disrupters", the study focuses on the inhibitory activity of natural product-kojic acid to PO. The results showed that kojic acid is a competitive inhibitor on the enzyme is from diamondback moth, Plutella xylostella.