Abstract: Objective:To prepare and study the properties of polysaccharide fractions from Dendrobium officinale(DOP),compare their in vitro immunological activity and discuss the structure-bioactivity relationship. Methods:Precipitation with different ethanol was used to prepare the Dendrobium officinale polysaccharide fractions. The sugar,uronic acid,protein content,monosaccharide composition,and molecular weight of these fractions were analyzed. Their in vitro immunological activity were evaluated with macrophage RAW264.7. Results:The results indicated that DOP,F30,F40,F50 and F50S were all neutral sugars. The uronic acid contents of these fractions were lower than that of DOP,F50S had the highest protein content. The molecular weight of DOP,F30,F40,F50,and F50S were 262.4,314.5,248.3,202.9 and 136.2 kDa,respectively. DOP,F30,F40,F50,and F50S were mainly composed of mannose and glucose,with the ratio at 5.16∶1,3.67∶1,4.16∶1,5.62∶1 and 5.86∶1,respectively. DOP and its fractions could significantly(p<0.05)enhance the proliferation capacity of RAW264.7 macrophages,release of NO,and secretion of TNF-α and IL-1β. Conclusion:The polysaccharide fractions of Dendrobium officinale had the capacity of immune regulation,and the immunomodulatory activity of small molecular weight is stronger.