•Fine grinding yeast cell wall (YCW) stimulated immunoglobulins secretion.•Fine grinding YCW improved intestinal oxidative status.•Particle size mediated the immune and antioxidant function of YCW. The current study was conducted to investigate the effects of yeast cell wall (YCW) powder processed to different particle sizes on the growth performance, serum metabolites, immunity and antioxidant status of broilers. A total of 144 one-day-old Arbor Acres broiler chicks were allocated into 3 dietary treatments consisting of 6 replicates with 8 chicks per replicate, and received a basal diet and basal diet supplemented with either 1g/kg coarse or fine grinding YCW powder until 42 days of age, respectively. Treatments did not affect growth performance of broilers. Compared with the control group, total cholesterol content in serum (P=0.032) and bursa weight (P=0.019) was increased by the inclusion of YCW at 42day. Fine rather than coarse grinding YCW powder increased ileal secretory immunoglobulin A (SIgA, P=0.040) and immunoglobulin G (IgG, P=0.019) content at 21day. Additionally, birds fed diets containing fine grinding YCW showed increased ileal total superoxide dismutase (T-SOD) activity (P=0.049) at 21day whereas decreased malondialdehyde (MDA) accumulation at 42day (P=0.021), respectively. In conclusion, the ability to improve immune function and intestinal oxidative status of broilers was more pronounced in fine grinding YCW powder.