Gracilaria lemaneiformis is cultivated on a large scale in China for industrial production of agarose, a natural polysaccharide, which has been shown to have many beneficial bioactivities such as antitumor, antiviral antioxidant activities, etc. In the present study, the hypoglycemic and antioxidant effects of a polysaccharide extracted from Gracilaria lemaneiformis (GLP; Mw, 121.89 kDa) and its chemically degraded products (GLP1 and GLP2: Mw, 57.02 and 14.29 kDa, respectively) were investigated in alloxan-induced diabetic mice. The intragastric administration of GLP, GLP1 and GLP2 for 21 days induced an obvious decrease (P < 0.05) in blood glucose levels in comparison with untreated diabetic mice. Furthermore, GLP, GLP1 and GLP2 caused evident increases (P < 0.05) in both ant i-oxidase (SOD and GSH-Px) activities and the total antioxidant capacity (T-AOC) and a significant decrease (P < 0.05) in the level of malondialdehyde (MDA) in the liver, pancreas and kidney of diabetic mice. Even though GLP, GLP1 and GLP2 did not show any significant difference in the structure and sulfation levels, GLP1 demonstrated more potent effects than GLP and GLP2 at the same dose. Histopathological examination of the pancreas and kidney revealed that the damaged tissues induced by alloxan were repaired to a certain degree after the treatments of GLP, GLP1 and GLP2.