The objectives of this study were to determine the effect of osthole on the insulin resistance (IR) in high-fat and high-sucrose-induced fatty liver rats and to investigate its potential mechanisms. The rat model was established by orally feeding high-fat and high-sucrose emulsion by gavage for 9 weeks. The experimental rats were treated with osthole 5 and 10 mg/kg, lipanthyl 30 mg/kg, and rosiglitazone 4 mg/kg after oral high-fat and high-sucrose emulsion for 6 weeks and were sacrificed 4 weeks after administration. The total cholesterol (TC), triglycerides (TG), and free fatty acids (FFA) in serum and hepatic tissue, fasting blood glucose (FBG), fasting serum insulin (FINS), serum adiponectin, and liver weight were measured. The homeostasis model assessment of insulin resistance (HOMA‐IR) and coefficient of hepatic weight were calculated. The results showed that after treatment with osthole, the serum levels of TC, TG, and FFA, the contents of TG and FFA in hepatic tissue, and body weight gain were lowered, especially in the osthole 10 mg/kg group (p < 0.05 or p < 0.01). Moreover, the histological evaluation of liver specimens demonstrated that the steatosis and inflammation in liver in osthole-treated groups were improved, especially in the 10 mg/kg group (p < 0.05). Importantly, the levels of FBG, FINS, and HOMA‐IR in the osthole 10 mg/kg group were decreased (p < 0.01), while the level of serum adiponectin in the osthole-treated groups, like PPARα agonist lipanthyl and PPARγ agonist rosiglitazone, was increased (p < 0.05). These results revealed that osthole could improve the IR induced by high-fat and high-sucrose emulsion in fatty liver rats, and its mechanism might be associated with increment of adiponectin release via activation of PPARα/γ pathway. [ABSTRACT FROM AUTHOR]