The production of many edible and medicinal mushrooms has been steadily increasing, because of several of their physiological effects. In the present study, we investigated the antihyperglycemic activity of large exopolysaccharide molecules (PSF), which are produced in the fermented broth of ferula mushroom Pleurotus ferulae, on streptozotocin-induced diabetic rats. All experimental rats were divided into 6 groups consist of 8 rats. The diabetic rats were fed a diet containing PSF for 6 weeks at a dose of 30 mg (PSFL-group), 90 mg (PSFM-group), or 250 mg/kg body weight (PSFH-group) daily, respectively. The fasting blood glucose level of the PSFH-group was the lowest among all 3 PSF-fed groups. Insulin levels increased and HbA1c levels decreased significantly for the three PSF-fed groups in comparison with negative control group during period of breeding. The PSFH-group’s low-density lipoprotein cholesterol and triglyceride levels were lower than those of other groups. A dose-dependent effect revealed that the exopolysaccharide of P. ferulae might mitigate hyperglycemia at the highest dose of 250 mg/kg body weight.