The present study examined the effect of the king oyster mushroom (Pleurotus eryngii) on insulin resistance and dyslipidemia in db/db mice. Four-week-old db/db mice were fed an AIN-93G diet or a diet containing 5% king oyster mushroom for 7 weeks. The blood glycated hemoglobin and serum glucose levels in the mushroom group were significantly lower than the control group (p<0.05). Dietary king oyster mushroom significantly reduced the homeostasis model assessment for insulin resistance (HOMA-IR), total cholesterol, and triglyceride, andincreased high density lipoprotein (HDL)-cholesterol. These results indicate that king oyster mushroom improves insulin sensitivity and exerts anti-hyperglycemic and anti-hyperlipidemic effects in db/db mice.