The fibroin genes of domestic silkworm, Bombyx mori were so strongly transcribed that the amount of fibroin proteins reaches up to 20% of total weight in the fifth instar. This strong expression is believed to be regulated in the transcriptional level due to their promoter. Because of this powerful expression system and well-known characteristics of silkworm, the application of transgenic technique to silkworm will provide an excellent opportunity to mass-produce of biomolecules. In this study, P element vector of Drosophila melanogaster was microinjected into eggs and mediated stable germ-line transformation of the lepidopteran insect, B. mori. The insertion and stable inheritance of transgene was confirmed by PCR sequencing, Southern blotting and inverse PCR. This method of stable gene transfer in B. mori of P element will provide the way for the basic research and biotechnological applications in lepidopteran insects.