This study investigated the effects of dietary mannan oligosaccharicles (MOS) on growth, body composition, and small intestine and liver histology of rainbow trout (Oncorhynchus mykiss Walbaum). Experimental diets were supplemented with 0 (control), 1.5, 3.0, or 4.5 g MOS per kg commercial trout feed and randomly assigned to treatment groups. Enhanced growth performance was generally observed in fish fed the diet supplemented with 1.5 parts per thousand MOS. Villi of fish fed diets supplemented with 1.5 or 3.0 parts per thousand MOS were longer than those of fish fed 4.5 parts per thousand or no dietary MOS (p<0.05). Protein contents increased as the rate of MOS increased (p<0.05). There were no significant differences in feed conversion ratio, protein efficiency ratio, or hepatosomatic index (p>0.05) and MOS had no detrimental effects on the intestine.