A solitary endoparsitoid, Cotesia plutellae, has been regarded as a major biological regulator to manipulate field population of diamondback moth (DBM), Plutella xylostella. It parasitizes DBM and alters its physiology into a favorable condition for the parasitoid development. This research has been focused on the physiological changes in terms of internal morphology and biochemical changes of the parasitized DBM. The parasitized DBM exhibited significantly hypotrophied structures in Malpighian tubules, fat body, and testes, while it did not show apparent change in the digestive organ structure. The parasitoid represented almost 70% of total body weight of the parasitized DBM. This estimate was well corresponded to the measurements of the major nutrient amounts in the parasitized DBM. This study clearly indicates that the major nutrients obtained by the parasitoid DBM are exploited by the koinobiont parasitoid for its own development.