The aim of the present study to isolate the water-soluble polysaccharide from Astragalus membranaceus roots (AMP) and investigate the structural effects on RAW 264.7 murine macrophage and natural killer (NK) cells. AMP mainly consists of carbohydrates (66.2 %), proteins (11.8 %), and sulfates (18.0 %) with minor level of uronic acid (2.0 %). The structural modification was carried out by removal of protein and sulfate from AMP through the deproteination and desulfation. After deproteination (DP), the protein content was decreased from 11.8 % to 5.4 %. Similarly, the sulfate content of desulfated AMP (DS) was decreased from 18.0%-8.1%. AMP and DP could stimulate RAW264.7 cells to produce nitric oxide (NO) and up-regulate mRNA expression through NF-kappa B and MAPKs pathways. However, DS showed a considerably lower level of NO production than AMP and DP, suggesting that DS could not stimulate RAW264.7 cells. AMP and its derivatives significantly increased the natural killer cells (NK cell) proliferation (113.1%-128.7%) and cytotoxicity against HeLa cells (37.4%-55.5%). However, DS showed the lowest level of NK cells activation through the expression of IFN-gamma, TNF-alpha, Granzyme-B, and NKp44. These results suggest that sulfate groups of AMP might play a crucial role in the RAW264.7 cells and NK cells activation.