Objective：To study the chemical stability of leflunomide in a wide pH value range. Methods：A method to determine leflunomide by high performance liquid chromatography (HPLC) was established. The rate constants at different temperature and different pH values were determined. The pH values of maximum stability at different temperatures and the decomposed activation energy values for different pH values were obtained. Results：The pH values of maximum stability were pH2.85-4.56 at 20-40℃, the activation energy also showed maximum values within pH2.85-4.56. The most stable pH values of leflunomide aqueous solution moved to lower pH values with a rise in temperature. When temperatures were over 60℃, the pH values of maximum stability were below 1.80. Conclusion：The chemical stability of leflunomide in aqueous solution can be improved by modifying pH values.