The present study investigated the effects of prebiotic mannan oligosaccharide (MOS; activeMOS ® ) on the growth performance, intestinal microflora, body composition, haematological and serum biochemical parameters in rainbow trout juvenile (Oncorhynchus mykiss). After acclimation, fish (1.67±0.006 g) were allocated into 12 tanks (25 fish per tank) and triplicate groups were fed a control diet or diets containing 1, 2.5 and 4.0 g MOS kgG 1 . At the end of trial (60 days), blood samples were collected from caudal vein of 48 apparently healthy fish. Result showed that in level of 1 g MOS kgG 1 acquired final weight, Body Weight Increase (BWI), Specific Growth Rate (SGR) and Feed Conversion Ratio (FCR) were significantly higher than the other experimental groups (p<0.05). The highest and the lowest growth performances were observed in 1 g MOS kgG 1 and control, respectively. The fish fed with 1 g MOS kgG 1 had highest but not significant (p>0.05) intestinal lactic acid bacterial count. By increasing the supplementation level of MOS, the content of crude protein increased (p<0.05). The result showed no significant differences in serum enzymes (ALT, AST, ALP) activity, glucose, cholesterol, triglyceride and total protein among treatments (p>0.05) but levels of lymphocyte in the group fed 1 g MOS kgG 1 was significantly higher than other groups (p<0.05). Also, a non significant elevation of White Blood Cell (WBC), haematocrit, hemoglobin and eosinophil levels was found in the fish fed diet 1.0 g MOS kgG 1 (p>0.05). The results indicated that dietary administration of mannan oligosaccharide at the level of 1 g kgG 1 can positively influence the growth performance, intestinal microflora, body composition and some blood parameters of juvenile rainbow trout.