To evaluate the effects of altitude on free radical scavenging potential and total reducing capacity of the medicinal plant Achillea wilhelmsii C. Koch, aerial parts of yarrow were collected at the flowering stage from four altitude of Dena (2448, 1890,  1800and 1739 m) and three altitude ofBoyer –Ahmad(2373, 1793 and 1485 m) districts located in Kohgiluyeh and Boyer-Ahmad province (latitude 30.67 N, longitude 51.60E). Free radical scavenging potential and total reducing capacity of methanolic extracts of the plant samples were determined by DPPH (1,1-diphenyl- 2-picrylhydrazylradical) and Folin-Ciocalteu assays, respectively. Free radical scavenging potential expressed as µmol Trolox equivalent g-1 DW (µmol TE g-1 DW) ranged from 508.27 to 155.50. Samples collected from the lowest altitude of Dena and Boyer-Ahmad-districts (samples “D” and “G”) showed the highest free radical scavenging potential. Total reducing capacity ranged from 65.15 to 51.83 mg gallic acid equivalent g-1 DW (mg GAE g-1 DW). Samples collected from the lowest altitude of Boyer-Ahmad district (Samples “G”) showed the highest total reducing capacity with 22.6 percent increase compared to the highest altitude in this region. Although free radical scavenging potential and total reducing capacity both increased with reduced altitude in Boyer-Ahmad, such close relationship was not observed in Dena district. It is concluded that free radical scavenging potential and total reducing capacity of the medicinal plant yarrow is affected by altitude and lower altitude may favor higher antioxidant capacity.