Background: Overproduction of uric acid by xanthine oxidase (XO) causes gout. XO inhibitors such as allopurinol, are the most important available anti-gout drugs. Medicinal plants are available natural sources that may be useful for the treatment of gout. Objective: In this study, the XO inhibitory activity of aqueous extracts of Phaseolus vulgaris, Cinnamomum zeylanicum, Mentha longifolia, Cichorium intybus, Capparis spinosa and Trigonella foenum-graecum that their anti-gout effects have been reported in the literature, were measured. Methods: In these experiments, under controlled conditions xanthine turns into uric acid by XO. Uric acid absorbance was measured at 295 nm using a UV spectrophotometer. Adding allopurinol (as positive control) or aqueous extracts to the solution containing XO, can decrease uric acid production by inhibition of this enzyme. At first, XO inhibitory activity of allopurinol and reproducibility of the method were evaluated by conducting three experiments. After that, the XO inhibitory activity of aqueous extracts at 0.1, 0.5, 1, 1.5, 2 and 3 mg/ml were measured. Results: The results showed an EC50= 0.38 μg/ml for allopurinol. The obtained data showed that Mentha longifolia in compare with its control could inhibit enzyme up to 72% (p< 0.001) at 3 mg/ml. Maximum XO inhibitory activity of Phaseolus vulgaris at 3 mg/ml in compare with its control was 27% (p< 0.001). Other extracts did not have any significant effect on XO. Conclusion: The results showed that part of the anti-gout effects of Mentha longifolia and Phaseolus vulgaris is due to XO inhibition.