The study on free amino acid variation in the muscle and hemolymph of itopenaeus vannamei in different salinities revealed that total free amino acids of the hemolymph were 87.58 mg/ml, 569.27 mg/ml, 1 038.68 mg/ml respectively and total free amino acids of the muscle were 2 250.13 mg/100 g, 2 330.42 mg/100 g and 2 548.74 mg/100 g,respectively, when salinity was 0, 15 and 30 . As salinity increased, total free amino acids of the hemolymph of L. vannamei increased significantly (P<0.05). The concentrations of free Gly, Glu, Arg, Ala in the hemolymph ofL. vanamei increased sharply, with increased salinity, in the meanwhile the concentration of free Gly, Glu, Arg, Ala in the muscle of L. vanamei correspondingly increased. However, the increase of concentrations of free Gly, Glu, Arg, Ala in the muscle of L. vanamei was less than those in the hemolymph. Free amino acids Gly, Glu, Arg, Ala played an important role in regulating osmotic pressure.