Total phenolic contents, hydroxyl radicals and one 1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activities, antioxidant capacities and reducing powers of Sideritis leptoclada, which is an endemic plant to Turkey, and Mentha x dumetorum were investigated in water extracts. Total phenolic contents were detected to be 0.32 mu g gallic acid/mg dry biomass in the presence of 1.33 and 9.82 mu g/mL, respectively. Fifty percent scavenging activities of hydroxyl and DPPH radical were determined with 0.03 and 0.16 mg/mL extracts of S. leptoclada and 0.21 and 1.6 mg/mL extracts of Mxdumetorum. In addition, total antioxidant capacities of S. leptoplada were also higher than Mxdumetorum and there was a significant difference between the controls. The result of this study suggests that extracts of S. leptoclada and Mxdumetorum can be used as potential source of natural antioxidants and as a possible food supplement or in the pharmaceutical industry.