[Purpose] Functional instability leads to a delay in the muscle reaction time and weakness of the peroneal muscles. The present study examined the effects of transcutaneous electrical nerve stimulation during balance exercise on patients with functional instability of the ankles, including the ability to land after jumping at the center of foot pressure. [Subjects] The subjects were seven males with a history of ankle sprain. All had a sprained ankle score of ≤80 points on Karlson’s functional instability test. [Methods] They were asked to jump over a 20-cm-high platform sideways for 10 consecutive seconds on a force plate with one leg. The length of the center of pressure was measured for comparison of balance exercise and balance exercise with simultaneous transcutaneous electrical nerve stimulation. [Results] The length of the center of foot pressure on the sprain side was significantly greater than on the non-sprain side under both conditions. Under the balance exercise with simultaneous transcutaneous electrical nerve stimulation therapy condition, the length of the center of foot pressure on the sprain side was significantly reduced, with the values being 627.0 ± 235.4 and 551.8 ± 171.1 mm before and after the challenge, respectively. [Conclusion] Ankle instability on the sprain side was significantly reduced under the balance exercise with simultaneous transcutaneous electrical nerve stimulation therapy condition before and after the challenge. Peroneal muscles showed increased activity caused by common peroneal innervation.