A feeding experiment for 160 days was conducted to evaluate effects of dietary soybean meal (SBM) on the apparent digestibility, ammonia excretion and body composition of rainbow trout (Oncorhynchus mykiss). Seven diets containing $0~70\%$ SBM were formulated based on iso-nitrogenous and iso-caloric basis of $44\%$ crude protein and approximately 4,200 Kcal/kg of gross energy. To evaluate the nutritional utilization of the SBM diets at the end of 60 and 120-day rearing with test diets, digestion rates of protein and lipid of SBM diets were measured. Fish fed the diets containing above $46\%$ SBM showed higher apparent digestibility for protein, but lower for lipid than did fish fed the control diet in both trials. Protein and fat contents in the carcass were similar for all experimental fish, except for fish fed $58\%$ and $70\%$ SBM which showed lower fat content than the others, but composition of fatty acid and amino acids were not affected by dietary SBM levels. Gill and urinary post-prandial ammonia ($NH_3-N$) excretions were measured at 12 and 24 hours after single feeding of the experimental diets. Excretions of $NH_3-N$ measured after 24 hours were almost 1 times higher than those measured after 12 hours. Total excretion fer the 24 hours by fish fed $70\%$ SBM was $15\%$ higher than that of fish fed the control diet. Results of present study may suggest that the rainbow trout which were fed above $34%;or;46\%$ of SBM diet showed a decrease gradually in lipid bioavailability compared to the control group.