This study was carried out with an aim of effect of modified chitosan with vitamin C on growth and non-specific immunity in rainbow trout ( Oncorhynchus mykiss ) fingerlings. For this purpose, six different food diets include; basal diet, a diet containing common chitosan with 0.25 %, diets containing modified chitosan (0.10 %, 0.25 % and 0.50 %) and a diet containing basal diet and vitamin C, were assigned in three replications for a six week period. Mean (± standard error) weight of fish was 3.8 ± 0.2 g in the beginning culture period. The growth parameters were investigated, including of final weight, condition factors (CF), specific growth rate (SGR) and food conversion ratio (FCR). At the end of the experiment, all groups were exposed to hyper oxygen stress and relative percent survival (RPS) was studied. The results show that SGR and CF had significant differences among different treatments (P?0.05). The FCR did not significant differences among different treatments (P?0.05). The immunological parameters such as serum complement, lysozyme and bactericidal activities and intestinal microbiota were investigated. The lysozyme activities of blood serum had not significant differences among treatments (P?0.05) and were decreased at the end of experiment. The hemolytic complement activity had not significant after 15-day culture (P?0.05), while had significant difference at the end of the culture period (42-day) (P?0.05), with the highest level in the diet containing modified chitosan of 0.1 %. The addition of modified chitosan in the diet of rainbow trout fingerlings had the positive effect of serum bactericidal activity and had significant differences among treatments (P?0.05). Bacterial count shows that modified chitosan had no positive effect on total viable and lactic acid bacteria of the intestine. Also, results showed that rainbow trout fed on modified chitosan 0.25 % had the highest resistance to hyper oxygen stress. Based on some immunological parameters and relative percent survival values could be illustrated that the use of modified chitosan in the diet of rainbow trout fingerlings improved the non-specific immunity system during 42-day culture and increased the resistance against hyper oxygen stress. Keywords : modified chitosan, vitamin C, rainbow trout, innate immunity 59