Silkworm BmN4 cells were transfected with the helper plasmid and the piggyBac vector( piggyBac-FLAG) in which was inserted with the spider flagelliform silk expression cassette. Via antibiotic selection, most cells showed stable DsRed-expression. Immuno blot analysis showed that flagelliform silk protein of spider was expressed stably in BmN4 cells. Circular dichroism spectra indicated the existence of β-turn structure in recombinant spider flagelliform protein. The present results suggested that transgenic silkworm directly secreting functional spider silk protein in cocoon by using piggyBac system is feasible.