BACKGROUND: It is necessary to potentiate the immune system of fishes against stresses in farms. OBJECTIVES: This study was carried out to address the potential effect of Levamisole on immune system of rainbow trout against density stress. METHODS: 1500 fish (average weight of 50 g) were divided into 5 test groups, in which each test group was repeated three times with average density of 33 kg/m3. They were fed with commercial diet supplemented with Levamisole at concentrations of 0 (control), 100, 250, 500 and 1000 mg / kg for a period of 45 days. The fishes of all groups were then fed with Levamisole free diet and exposed to 2 and 3-fold density stress for the following 15 days. Blood samples were collected on days 0, 15, 30, 45 and 60 to evaluate the serum compliment and lysozyme activity as well as total immunoglobulins. RESULTS: The results showed that all used concentrations of Levamisole just had significant effect on compliment activity after 45 days feeding period (p<0.05). Higher levels of lysozyme and complement activity as well as total immunoglobulin were observed in 1000 mg/kg Levamisole treated group when exposed to density stresses 2 and 3-fold at the end of trial (day 60) (p<0.05). The highest overall survival was found in group which was treated with 1000 mg/kg of Levamisole. CONCLUSIONS: Our results revealed that using 0/1% Levamisole as an immunostimulator in commercial diet could potentiate rainbow trout against outbreak of high density stresses and increase its overall survival.