In this review,the direct effects of probiotics on the innate immune system of fish are discussed. Probiotics, such as frectooligosaccharide, mannanoligosaccharide, inulin, or glucan, are called immunosaccharides. They directly enhance innate immune responses including : phagocytic activation, neutrophil activation, activation of the alternative complement system, increased lysozyme activity, and more. Immunosaccharides directly activate the innate immune system by interacting with pattern recognition receptors (PRR) expressed on innate immune cells. They can also associate with microbe associated molecular patterns (MAMPs) to activate innate immune cells. How-ever, the underlying mechanisms involved in innate immune cells activation need to be further explored.many studies have indicated that immunosaccharides are beneficial to both finfish and shellfish.