The purpose of this study was to explore the effect of unilateral ankle instability on walking energy expenditure. Chronic ankle instability group consisted of 20 subjects their age average (21.4±5.55) years, height average (177.85 ± 4.96) cm and weight average (81.25 ± 6.87) kg, they were referred from orthopedic specialist. The control group consisted of 20 normal subjects, their age average (24.65 ± 4.61) years, height average (178.7 ± 3.76) cm and weight average (80.4 ± 12.43) kg. Participants walked on a treadmill at two speeds 5 km/h (comfortable walking) and 6.5 km/h (fast walking) for three min with resting period of twenty minutes between each test. Before testing there was 3 min warm up at 1.5 km/h, and after each test there was 3 min recovery at 1.5 km/h. The oxygen consumption and energy expenditure at the two speeds are measured by using ZAN 100 flow handy П medical device with a PC- connected to open spirometry system. Results revealed that there was a significant increase in oxygen consumption and energy expenditure of the two groups as walking speed increased from 5 to 6.5 km/h. There was no significant difference in oxygen consumption and energy expenditure between both groups at walking speed 5 km/h. However, the oxygen consumption and energy expenditure of chronic ankle instability group was significantly higher than control group at walking speed 6.5 km/h. So, chronic ankle instability increased the walking energy cost especially during fast walking.