Abstract: NO2-N is one of the main contaminations and a great stress factor in the intensive prawn culture system.Therefore it is very important to study the toxicity of NO2-N on Litopenaeus vannamei and effects of NO2-N on factors relating to the anti-disease ability for the health culture of L.vannamei.Acute toxicity test of NO2-N to L.vannamei was done by the usual method of biological toxicity test at different salinity.According to the result of acute toxicity test, NO2-N was introduced into the culture system of L.vannamei at tow levels(4mg/L,8mg/L).Those without introducing NO2-N were used as controls.The water in the culture system was changed with proportion of 100% every day.NaNO2 was introduced into the same system with the same con2 centration of NO2-N.Without renewing water, two experiment group were set.The filth of group 1 was cleared once every 4 days, while group 2 was not.The blood serumof prawn was drawn out on day 18 and the changes of the hemocyte number, the content of blood serum protein and the activity of phenoloxidase, superoxide dismutase, antibacterial enzyme, lysozyme of L.vannamei were tested.The result shows that the salinity had remarkable effect on the toxicity of NO2-N.When the salinity was 31 in the culture system, the 24h LC50,48hLC50,72hLC50,96hLC50was 3.419mg/L,175.3mg/L,100.2mg/L and 89.0mg/L, respectively: when the salinity was 17, the corresponding concentration was 132.3mg/L, 65.6mg/L,51.3mg/L and 39.5mg/L, respectively.All LC50 of the experiment group with salinity of 31 was significantly higher ( P 2-N was greater under the condition of lower salinity.NO2-N showed obvious effect on the anti2 disease ability of L.vannamei.Under water renew condition, and the NO2-N was 4mg/L and 8mg/L in the culture system, the hemocyte number, the content of blood serum protein and the activity of phenoloxidase, superoxide dismutase, antibacterial enzyme, lysozyme of L.vannamei were declined 66.3%, 6.5%, 21.9%, 18.9%, 24.8%, 48.9% and 68.3%, 11.6%, 43.9%, 23.9%,59.6%,56.9%, respectively in contrast to the control group.The activity of factors relating to the antidisease ability was significantly lower ( P 2-N and the higher NO2-N concentration is, the weaker anti2disease ability will become.