The objectives of this study were to assess the effects of supplementation of Astragalus membranaceus root powder (AMP) and AMP processed to different particle sizes on growth performance, antioxidant status, and serum metabolites of broiler chickens. The experiment was conducted with one hundred twenty 1-d-old Arbor Acres broilers in 5 groups of 4 cages and for both starter (0 to 21 d) and grower (22 to 42 d) phases. The treatments were basal diet only (control) and basal diet supplemented with 5 g/kg of diet of AMP processed to particle sizes of 300, 149, 75, or 37 µm. Average daily gain, ADFI, and feed conversion rate (FCR) were determined weekly, and carcass yield, serum antioxidant enzyme activity, and metabolites were determined at 21 and 42 d of the experiment. Supplementation of AMP increased (P < 0.01) activities of total superoxide dismutase (TSOD) and glutathione peroxidase (GSHPx), but reduced (P < 0.01) concentrations of malondialdehyde (MDA) and cholesterol in the serum of chickens at 21 and 42 d. Reducing AMP particle sizes from 300 to 37 µm linearly increased (P < 0.01) TSOD and GSHPx activities at 21 and 42 d, but linearly decreased (P < 0.01) MDA at 42 d. Concentrations of total protein, albumin, and globulin in the serum were also increased (P < 0.05) or tended to be increased (P = 0.05 to 0.10) by AMP and linearly increased (P < 0.01) as the AMP particle sizes decreased. However, both treatments had no effect on ADG, ADFI, or FCR throughout the entire experiment period, although carcass yield increased (P < 0.05) at 42 d. Dietary supplementation of AMP at the concentration of 5 g/kg of diet enhanced serum antioxidant status and its efficacy linearly increased as the AMP particle size decreased from 300 to 37 µm, but had no effect on growth performance of broilers.