A viral histone H4 (=CpBV-H4) is encoded in a polydnavirus, Cotesia plutellae bracovirus, and symbiotically associated with an endoparasitoid wasp, C. plutellae. It has an extended N-terminal tail consisting of 38 amino acid residues, compared to the host H4 and this extended N-terminal tail has been postulated to play a crucial role in an epigenetic control of gene expression. The (SSH) suppression subtractive hybridization analysis was analyzed in transcriptome by short-read sequencing technology. The SSH analysis provided several target and nontarget genes of a viral histone H4. In this study, we analyzed the effect CpBV-H4 on the expression of two target genes serpins and histone lysine N-methyl transferase. Transient expression of CpBV-H4 by microinjecting recombinant expression vector to non parasitized larvae of Plutella xylostella showed that it was expressed up to 70 h. Under this transient expression condition, we analyzed the effect of CpBV-H4 on the expression of target genes by RT-PCR at different time points. Interestingly, the CpBV-H4 significantly inhibited the expression of target genes after 44 h, while the truncated CpBV-H4 deleting the N-terminal tail did not show the inhibitory activity.