Litopenaeus vannamei head autolysate was consecutively extracted through ultrafiltration membrane with molecular weight cut offs (MWCO) at 3 000, 5 000 and 8 000 u, respectively. The distribution of relative molecular mass, composition of amino acid as well as ACE (Angiotensin I converting enzyme) inhibitory activity in different filtrates were measured. The results showed that ACE inhibitory activity of autolysate increased after extraction by extra fine membrane. Obviously, the inhibitory activity of the filtrate of 3 000 u extra fine membrane was 0.61 mg/mL, which ranked the first compared with other filtrates. Meanwhile, the most of macromolecular substance was retained by 8 000 u while the extracted peptides with relative molecular mass less than 3 000 u increased markedly with the decline of MWCO of membrane. In addition, the contents of aromatic amino acid and branched chain amino acid in filtrate of 3 000 u were higher than those in other filtrates.