Delayed arteriovenous (AV) fistula after soft tissue reconstruction with flap surgery is a rare complication. Here, we describe a case of delayed AV fistula formation after 4 years of reverse flow sural island flap surgery in the lower leg. The patient had swelling, tenderness, and color change to dark purple on previous flap area and foot, similar to cellulitis. Thrill and murmur were observed. AV fistula was formed around the previous vascular pedicle area, as revealed in angiography. We excised the right posterior tibial artery-saphenous vein fistula. The patient is having satisfactory progress since the surgery. We suggest that AV fistula was caused by enhanced angiogenesis and vascular damage.