Changes in the fat body structure of the host Plutella xylostella (Lepidoptera: Plutellidae) larvae induced by different parasitism factors of the parasitoid Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae) in different parasitized states were observed with light microscopy and transmission electron microscopy. The results revealed that the fat body cytology of parasitized larvae was different from those of pseudo-parasitized host and non-parasitized control. The polydnavirus (PDV) and venom had no obvious effects on the fat body structure in the pseudo-parasitism condition, where the fat body maintained the integrality with normal stacks of endoplasmic reticulum and mitochondria, glycogens accumulated in some cells, and more amounts of total lipids present but smaller as compared with those of the non-parasitized host larvae. The most striking features of fat bodies in the parasitized larvae were the cytoplasmic organelles decreased and mitochondria with ill-defined cristae, and especially during the final parasitism stage (when parasitoid larvae completed their development), the fat body structure was severely destroyed. In contrast, when the non-parasitized larvae were in the final phase of the 4th instar, the fat body cells developed normally and were undergoing transformation from the larval into prepupal stage, with large lipid bodies and rich mitochondria and glycogen present, some of them existing in free cell rosettes. These results suggest that different parasitism factors, i.e. PDV, venom, teratocytes and parasitold larvae, have different effects on the host fat bodies. Plutella xylostella (Plutellidae) : Host Cotesia plutellae (Braconidae) : Parasite General morphology; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Cotesia plutellae; Lepidopteran hosts; Parasitism factors effect on host fat body structure; Plutella xylostella Plutella xylostella; Fat body; Hymenopteran parasites; Parasitism factors effect on fat body structure; larvae; Cotesia plutellae none