Abstract: Four groups with different nitrate nitrogen (NO₃^--N) concentrations (4.24 mg · L^-1, 35 mg · L^-1, 70 mg · L^-1, 105 mg ·L^-1and 140 mg · L^-1) and a control group were set to study the effect of nitrate nitrogen on immune factors in serum of Pacific white shrimp (Litopenaeus vannamei). The activities of prophenoloxidase (PO), superoxide dismutase (SOD), alkaline phosphatase (AKP), acid phosphatase (ACP), lysozyme (LSZ) and nitric oxide synthase (NOS) in the serum were determined. Results show that NO₃^--N concentration of no more than 140 mg · L^-1 had effect on the activities of SOD, PO, ACP, AKP, TNOS, iNOS and LSZ in a short time, which weakened gradually along with time. At 48^th hour, the activity of iNOS in other groups had no significant difference, except the 35 mg · L^-1 group.