Abstract: Abstract Aim： To explore the relationship between structure and immunological activity of marine polysaccharide YCP，the physicochemical property and immunological activity of the YCP-derived fragment were studied. Methods： YCP was hydrolyzed by α-amylase from human saliva.The hydrolysate was purified to obtain an polysaccharide fragment by gel filtration chromatography.The physicochemical properties of this YCP-derived fragment was characterized by HPLC，FT-IR and TLC.In addition，changes of phagocytic activity，production of reactive nitrogen and macrophage binding were investigated. Results： The relative molecular weight of YCP-derived fragment was approximately 6.6×10³.The monosaccharide composition and FT-IR of the YCP-derived fragment were identical to YCP.No significant effect of the YCP-derived fragment on NO production and murine macrophage phagocyte were observed.And this fragment was not able to compete the binding between YCP and macrophages. Conclusion： The remarkable decrease of immunological activity of YCP-derived fragments degraded by α-amylase of human saliva suggests that the complete structure and high molecule weight of YCP are essential for its immuno-modulatory activity.