Probiotics are live micro-organisms which by improving intestinal microbial balance with health benefit the host, whereas, prebiotics are non-digestible oligosaccharides which by stimulating growth and microbial activity in intestin effects the well being of the host. The present study was carried out to investigate an intestinal microbial balance, growth performance, and immune responses of fish trough addition of Gallipro probiotic and Fermacto prebiotic to diet of rainbow trout, Oncorhynchus mykiss. In a completely randomized design with 3×3 factorial arrangements, 9 experimental diets counting three dietary levels of Gallipro (0, 0.2 and 0.4 g/kg diet) three levels of Fermacto (0, 1 and 2 g/kg diet) were used. Each treatment was replicated 3 times. Four-hundred and eighty six (486) rainbow trouts with mean live weight 19.06±1.83 g were randomly selected and allocated to 27 tanks (18 fish per tank). The experiment was carried out for 10 weeks. Results showed that 0.4 g/kg diet of Gallipro improved the specific growth rate, feed conversion ratio and weight gain at the end of experiment, but Fermacto showed the opposite effects on the similar criteria. The lowest triglyceride, cholesterol, HDL and LDL of fish blood serum were observed in the diet that contained 0.4 g/kg Gallipro, but application of Fermacto supplement showed a significantly increasing effect on the similar items. The highest increase in blood serum factors were observed in diet that contains 2 g/kg Fermacto. Gallipro and Fermacto combination in diets improved fish blood serum factors significantly (p 0.05). These supplements showed no significant effect on fish blood serum proteins. Gallipro and Fermacto caused significant differences in blood serum lysozyme activity. The diet containing 0.2 g/kg Gallipro and 1 g/kg Fermacto increased and 2 g/kg Fermacto alone decreased blood serum lyzosyme activity significantly. The presence of Bacillus subtilis in diet resulted in a higher bacterial population and improving fish intestinal microbial balance. The highest bacterial population in fish intestine were observed in the diet that contained 0.4 g/kg Gallipro and 1 g/kg Fermacto. Length of villous of fish intestinal epithelium was also affected by Gallipro and Fermacto. The highest length of villous was observed in the diet that contained 0.2 g/kg Gallipro. However, the lowest length was observed in fish receiving the diet supplemanted with 2 g/kg Fermacto. In addition, Gallipro and Fermacto supplementation improved significantly the length of villous.