Abstract: In this study we tried to measure Invertor and Evertor muscles strength with Isokinetic dynamometer to distinguish, if these muscles activate properly or not. Thirty subjects (15 healthy and 15 patients with chronic functional ankle Instability (CFAI) aged 18-30 years) participated in this study. Maximum eccentric and concentric peak Torque of invertor and evertor muscles measured with Biodex system in 2 different speeds, 60 and 120 degree per second. Statistical analysis showed that maximum eccentric peak torque of invertor muscles in both 60 and 120 degree per second differed statistical significance between two groups, as in the patients group was less than normal subjects (p<0.05). It means that eccentric weakness of invertor muscles will lead to CFAI.