Compartment syndrome (CS), a complication at the donor site of anterolateral thigh free flap (ALT-FF), has a low incidence. Our patient was a 26-year-old male, a professional soldier with large thigh muscles. His defect on the right ankle was reconstructed with a 30×10 cm2-sized ALT-FF. By the pinch test, the flap width was designed to be sufficient for primary closure of the donor. However, on the postoperative day 8, severe pain, tense and purulent discharge were found at the donor site. We diagnosed CS clinically, and immediately performed incision and drainage. There are various extrinsic or intrinsic factors that can influence the development of CS. Therefore, in patients with these contributing factors, such as a young man with high muscle mass, a higher level of suspicion for CS is needed. Moreover, it would be safer to design the flap width smaller than the pinch test or to cover the donor site with a skin graft.