This study describes the effect of oral administration of beta-glucan as an immunostimulant on phagocytic activity and antibacteria of orange-spotted grouper Epinephelus coioides. After 15 days feeding period with commercial diets containing beta-glucan (Macrogard, Biorigin) at 500, 1000 and 2000 ppm, phagocytic activity of head kidney leucocytes in fish fed diets was significantly higher than those of control group by 16.0 percent,26.0 percent and 20.7 percent respectively, compared to the control group with 13.3 percent. Furthermore, accumulative mortality rates of fish against Vibrio parahaemolyticus in treatment groups at 500, 1000, 2000 ppm beta-glucan were significantly lower than those of control group by 29.2 percent,20.8 percent and 20.8 percent respectively, compared to the control with 70.8 percent. However, both phagocytic activity and antibacterial of treatment groups and control group were not difference after 15 days stop feeding with beta-glucan.