The experiment was conducted to investigate the effects of different crude protein (CP) levels diet supplemented with exogenous protease on performance, serum biochemical indices and antioxidant function of broilers. A 3×3 factorial design with 3 CP levels (21%, 20% and 19% at 1 to 21 days, 19%, 18% and 17% at 22 to 42 days) and 3 protease levels (0, 150 and 300 mg/kg). High CP diet without protease was a positive control (PC) treatment, low CP diets were two negative control (NC1 and NC2) treatments. A total of 1 080 one-day-old Arbor Acres (AA) broilers were randomly assigned to 9 treatments with 6 replicates per treatment and 20 broilers per replicate (half male and half female). The experiment lasted for 42 days. The results showed as follows: 1) CP levels had significant effects on feed-to-gain ratio (F/G), average body weight (ABW), average daily gain (ADG) and average daily feed intake (ADFI) of broilers (P<0.05), when the dietary CP levels was reduced from PC to NC2, ABW and ADG were significantly reduced (P<0.05), F/G was significantly increased (P<0.05). Protease level had significant effects on F/G (P<0.05), 300 mg/kg protease supplementation significantly reduced F/G of broilers at 1 to 21 days. 2) CP levels had significantly effects on the content of albumin and globulin in serum (P<0.05). The serum albumin content of broilers at 42 days in PC treatment was significantly higher than that in NC2 treatment (P<0.05), and the serum globulin content was significantly higher than that of NC1 treatment (P<0.05). Protease level had significant effects on serum creatinine content (P<0.05), 300 mg/kg protease supplementation significantly reduced serum creatinine content of broilers at 21 days (P<0.05). The interaction between protease level and CP level had significantly effects on the content of total protein and globulin in serum of broilers at 21 days (P<0.05). 3) CP levels had significantly effect on serum glutathione peroxidase (GSH-Px) activity (P<0.05), the serum GSH-Px activity of broilers at 42 days in PC treatment was significantly higher than that in NC2 treatment (P<0.05), protease level had significantly effect on serum GSH-Px activity and malondialdehyde (MDA) content; compared with the group without enzymes, serum GSH-Px activity was increased significantly and serum MDA content was decreased significantly in 300 mg/kg protease supplementation treatment (P<0.05). In conclusion, when dietary CP levels is reduced 1%, 300 mg/kg protease supplementation has no effects on performance, serum biochemical indices and antioxidant function of broilers, and 300 mg/kg protease supplementation in basal diets has a better effect.