An endoparasitic wasp, Cotesia plutellae, inhibits pupal metamorphosis of diamondback moth, Plutella xylostella. The parasitized larvae extended their larval stage about 2 days at 25$^{circ}C$ and died after escape of wasp larvae for pupation. We set up a hypothesis that the wasp parasitization inhibits juvenile hormone (JH) degradation, which leads to extend larval stage and blocks metamorphosis. In this study, the activities of JH esterase (JHE), a major JH degrading enzyme in hemolymph, were analyzed by use of a surrogate substrate, HEPTAT (methyl 1-heptylthioacetothioate). Hemolymph JHE activities of P. xylostella showed a typical Michaelis-Menten enzyme kinetics in response to HEPTAT, where it exhibited K$\_$M/ at 19.7 ${mu}$M and $V_{max}$ at 228.8 ${mu}$M/min/${mu}$1. The parasitized C. plutellae did not exhibit decrease of the JHE activity, but rather kept high JHE activity during the extended larval period.