Antimicrobial peptide CM4, a small cationic linear alpha-helical peptide that consists of 35 amino acids, was isolated from Bombyx mori. To improve the expression level of CM4 in Escherichia coli, tandem repeats of CM4 gene were constructed and expressed as fusion proteins (TrxA-nCM4, n=1, 2, 3,...,8) by constructing the vectors of pET32-nCM4 (n=1, 2, 3,...,8). Comparison among the expression levels of soluble fusion protein TrxA-nCM4 (n=1, 2, 3,...,8) suggested that BL21 (DE3)/pET32-3CM4 was an ideal recombinant strain for CM4 production. Under the selected conditions of cultivation and isopropylthiogalactoside (IPTG) induction, the expression level of CM4 was as high as 68mg/l with about 21% of fusion protein in soluble form, which was the highest yield of CM4 reported so far.