Aim To isolate a complex polysaccharide (AMP-B) from Atractylodes macrocephala Koidz and study its phtsico-chemical properties and hypoglycemic activity. Methods The root of Atractylodes macrocephala K. was extracted with water and precipitated with ethanol, dialyzed against water and freeze-dried to get the crude polysaccharides (AMP). A complex polysaccharide (AMP-B) was isolated and purified on DEAE-cellulose column. The model of diabetes rats was established with alloxan injection through the tail vein. Male rats were divided into 5 groups: the normal group, the control group, and three AMP-B-fed groups. Measuring the blood glucose, water and food consumption, thymus and pancreas index, and studying cut sections of pancreas tissues. Results AMP-B is a complex-polysaccharide, elemental analysis of AMP-B shown C 32 84%, H 5 68%, and N 1 79%. The neutral polysaccharide content of AMP-B was 50 3%, uronic acid was 40 4%, and protein was 11 5%. Monosaccharide composition of AMP-B was determined by GC, AMP-B composed of Glc, Gal, Man, Ara and Rha in a molar ratio of 3 0∶2 5∶1 3∶3 5∶1 0. AMP-B was found to reduce blood glucose level in alloxan-diabetic rats markedly at doses of 50, 100 and 200 mg·kg -1 by ig, but no effect in normal rat. AMP-B was found to decrease the consumption of water and food, recover pancreas damage of diabetic rats obviously, inhibited the atrophy of thymus and pancreas of the diabetic rats induced by alloxan. Conclusion AMP-B showed significant hypoglycemic effect on the experimental hyperglycemias rats induced by alloxan.