Abstract: Abstract: We synthesized the code-optimized gene of Cecropin B according to it’s natural sequence by overlap extension PCR, The gene was then cloned into the vector pMD-18T and was confirmed by DNA sequence. In order to get the whole mammary specific expression cassette, the gene of Cecropin B was cloned into the mammary specific expression vector pIRES-Bcp-neo which contain the goat β-casein gene 5'flanking regulation sequence, The whole mammary specific expression cassette was cloned into the retrovirus vector pLNCX, so the retrovirus vector pLNCX-bCP-cecB was constructed, This vector was transfected into a packaging cell line PA317, RNA from the vector is packaged into infectious, replication-incompetent retroviral particles which was then injected directly into the lactating mammary gland of bovines. The expression and bactericidal activity was detected using inhibition zone assay for the bacteria: E.coli K12D31, Antibacterial Peptide Cecropin B expressed in milk possesses bactericidal activity which can last at least thirteen days.