A genome project has been launched and aims to sequence total genome of Cotesia plutellae bracovirus (CpBV). This on-going research has identified seven EP1-like gene (‘ELP’) in CpBV genome. This study was intended to analyze gene expression of these seven ELPs in the parasitized diamondback moth, Plutella xylostella. All CpBV-ELPs were expressed only in P. xylostella parasitized by C. plutellae during early parasitization period. However, their expressions were different in tissue specificity among hemocytes, fat body, gut and epidermis tested. CpBV-ELP1 showed a persistent and ubiquitous expression pattern and was selected for subsequent functional assay. A recombinant CpBV-ELP1 in a eukaryotic expression vector was injected into non parasitized P. xylostella. In the treated P. xylostella, CpBV-ELP1 expression was detected by RT-PCR and induced significant reduction in total hemocytes compared to larvae injected with the cloning vector alone. Expression knock-down using RNAi revealed that the cytotoxic effect was induced by CpBV-ELP1 expression. Here, we propose a novel function of CpBV-ELPs as a cytotoxic parasitic factor.