A dose-response experiment was conducted to investigate the impacts of dietary threonine (Thr) levels on growth performance, serum biochemical indices, antioxidant capacities, and gut morphology of broiler chickens. Four hundred and thirty-two 1-d-old commercial broilers were allocated to 4 treatments consisting of 6 replicates of 18 birds. The experimental treatments received the same Thr-deficient basal diet and were labeled as follows: 85%, 100%, 125%, and 150% of NRC (1994) recommendations. The results demonstrated that on 21 d and 42 d, average daily weight gain (ADG, 22 to 42 d, 0 to 42 d) increased quadratically or cubically as the inclusion of Thr increased, while feed conversion ratio (FCR, 0 to 21 d, 0 to 42 d) decreased quadratically or cubically as dietary Thr increase from 85% to 150%. Excess dietary Thr levels triggered plasma aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities. The concentrations of total protein (TP) and globulin (GLO) increased quadratically with increasing Thr level, and the highest concentrations of TP and GLO were obtained at the 125% Thr level. Moreover, the plasma uric acid (UA) concentration decreased linearly or quadratically with the increase in dietary Thr level. Likewise, the serum glutathione peroxidase (GSH-Px) and total superoxide dismutases (T-SOD) activities increased quadratically as dietary Thr increased, and the highest activity of GSH-Px was obtained at the 125% Thr level, while the highest T-SOD level occurred in the 100% Thr group. Gut morphology of birds showed significant response to different graded concentrations of Thr level. Villus height (VH), crypt depth (CD), and VH:CD ratio (VH/CD) were increased linearly or quadratically by Thr supplementation. Therefore, the present study suggests that the NRC (1994) recommendations Thr level that was optimum for growth performance, and 125% of the NRC (1994) recommendations Thr level had better effects on biochemical indices, antioxidant function, and gut morphology of broilers.