Abstract The intracellular polysaccharides (CLSP) were extracted from Lepista sordida mycelium in submerged culture followed by concentration and ethanol precipitation. The antioxidant activities of CLSP were evaluated both in vitro and in vivo. In vitro antioxidant assay, CLSP had noticeable scavenging activities on superoxide anion, hydroxyl radical and 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical. For antioxidant testing in vivo, different doses of CLSP were orally administrated over a period of 6 weeks in a d-galactose induced aged mice model. As results, CLSP significantly inhibited the formation of malondialdehyde (MDA) and raised the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in mice brains and serums in a dose-dependent manner. The results provide a reference for large-scale production of CLSP by submerged fermentation and suggested that CLSP had potent antioxidant activity and could be explored as a potential dietary supplement to retard aging and attenuate age-related diseases in humans.