This study was conducted to evaluate the effect of dietary prebiotic mannan oligosaccharide on growth performance, survival and resistance against environmental stress of rainbow trout (Oncorhynchus mykiss) larvae during a 6-week experiment. Basal starter diet (containing 50% protein, 20% lipid and 21.65 MJ/kg energy) were supplemented with 0 (control), %.0.1, %0.2, 0.3%, 0.4% and %0.5 in a totally randomized design trial in triplicate groups. After acclimation, the larvae (initial weight of 370 mg) were placed and fed into trough (250 larve per tank). At the end of the trial, growth factors, survival, nutrition and stress resistance such as high temperature (33 °C), acidity (Low pH pH=2, Hcl), alkalinity (High pH pH=12, NaOH) and high salinity (40 ppt) were assessed. Results revealed that fish fed by different levels of prebiotic mannan oligosaccharide displayed significantly higher growth performance , feed efficiency and survival rate compared to the control group and the best performance was achieved at the level of %0.3 (P<0.05). In addition, resistance rate to environmental challenge stress increased significantly in fish fed by diet supplemented with prebiotic mannan oligosaccharide compared to the control group specially at the level of %0.3 (P<0.05). The results indicated that the addition of %0.3 prebiotic mannan oligosaccharide to the starter diet of rainbow trout larvae can improve growth performance, feeding efficiency and increase in resistance to environmental stress and seemed to be an effective immunostimulant.