The study was to investigate effects of dietary Eucommia ulmoides Oliver (EUO) bark aqueous extract on growth performance and nutritional composition of rainbow trout (145.56 ± 4.12 g) (Oncorhynchus mykiss) fed five different diets, including 0 (EUO-0), 5 (EUO-5), 10(EUO-10), 20 (EUO-20) and 40(EUO-40) g kg-1 EUO bark aqueous extract. The muscle lipid of fish in EUO-40 was significantly higher than that in other groups (p < 0.05), and fish in EUO-0 showed significantly lower muscle crude lipid content than that in EUO-5, EUO-10, and EUO-20 groups (p < 0.05). Fish in EUO-40 had significantly higher muscle Phe, Gly, Ala, delicious amino acids contents than that in EUO-0 (p < 0.05), and fish in EUO-20 had significantly higher muscle Ala content than that in EUO-0 (p < 0.05). Fish in EUO-10 showed significantly higher 22:1n-9 content in muscle than that in EUO-0 (p < 0.05). Contents of 18:3n-6 in muscle of EUO-5 and EUO-40 groups were significantly lower than that of EUO-0 group (p < 0.05). Fish in EUO-40 had significantly higher 22:5n-3, 22:6n-3, and Σ PUFAS n-3 contents than that in other groups (p < 0.05). In conclusion, supplementation of EUO aqueous extract improved the nutritional composition of rainbow trout, and the supplemental level was estimated to be 40.00 g kg-1 diet.