Bacillus as novel micro-ecological agent has been widely used in livestock and poultry industry due to its robust resistance to extreme environment and the advantages of some probiotic effects, such as regulating intestinal microbial balance, promoting the digestion and absorption, enhancing animal immune function, and improving animal production. Our review focuses on effects of probiotic Bacillus on animal immune function covering animal immune organs, specific immune function, non-specific immune function and erythrocyte immune function. In addition, it shows the mechanism of probiotic Bacillus on animal immune function including immune regulation and synthesis of antimicrobial compounds. Furthermore, future research needs are proposed.