Abstract: To study the effects of threonine in low fish meal diet on growth, antioxidant capacity, digestive physiology, and intestinal inflammatory cytokines gene expression of triploid Oncorhynchus mykiss, five isonitrogenous and isoenergetic diets containing 0.45% (control), 0.76%, 1.09%, 1.29% and 1.64% threonine were formulated by adding L-threonine in the basal diet to feed fish with an initial body weight of (18.42±0.20) g for a 56-days trial. A total of 450 triutploid rainbow were randomly divided into 5 groups with triplicate groups of 30 fish each. Threonine at 1.09%, 1.29% and 1.64% level significantly increased serum superoxide dismutase (SOD) and and significantly decreased the content of malondialdehyde (MDA) (P<0.05). The highest activity of catalase (CAT) was in the 1.29% group, which was significantly different from that in control group (P<0.05). The digestive enzyme activities of intestinal lipase (LPS) and trypsin in diets supplemented with threonine level (0.76%—1.64%) were significantly higher than those in the control group (P<0.05) except threonine level at 1.64% in the activities of digestive enzymes. The intestinal morphology of 1.09% and 1.64% groups were well-developed in villi, orderly arrangement, and no fusion and shedding. The threonine levels had significant effects on IL-2, IL-8, IL-10, IgM, TNF-α and PepT1(P<0.05). With the increase of threonine level, IL-2, IgM and PepT1 showed a trend of increase first and then decrease, and the expression levels of IL-2, IgM and PepT1 reached the highest level in 1.29% and 1.09% groups. IL-8 showed a gradually increasing trend. IL-10 and TNF-α showed a decrease trend. The low fish meal diet (15%) supplemented with appropriate level of threonine improves the growth, digestive enzyme activity, intestinal morphology and structure development and immune enhancement of juvenile triploid Oncorhynchus mykiss.