The purpose of this study was to measure and analyze peroneus longus reaction time (PRT) as well as ankle movement during ankle sprain simulation. PRT was measured for six control ankles and six unstable ankles with an inversion ankle sprain (1 male and 5 females per group, respectively) using an ankle inverting platform and high speed camera. The unstable group showed a significantly slower PRT (58.8±8.7 ms) than the control group (46.5±8.1 ms). The inversion angular velocity was significantly faster in the unstable group (152.8±62.6 d/s) than the control group (83.2±38.4 d/s). There was no significant difference between the two groups regarding ankle eversion time. Our results indicate that it is important to lead an unstable ankle to reduce inversion angular velocity to prevent recurrent inversion ankle sprain.