Purpose: Chronic subtalar instability is not common and similar to chronic ankle instability and the incidence and cause chronic subtalar instability are not well known. Recently we have experienced chronic subtalar instability without chronic ankle instability which was treated with modified Brostrom procedures. Materials and Methods: The patient is 46 year old man who has suffered from left ankle sprain for 30 years and recently aggravated more than twice a day. On subtalar stress view, 14 degree angulation of subtalar joint was noted and on anterior drawer view, 8 mm anterior displacement of left ankle was seen. Results: In operation, there was no anterior talofibular ligament abnormility but calcaneofibular ligament loosening was found. Ligament reconstruction was performed using modified Brostrom procedure. At 12 months after operation, the patient complains no pain and no limit of motion and no instability. Conclusion: We experienced chronic subtalar instability without ankle instability treated with modified Brostrom procedures. No instability was found after treatment without complication.