In situ bypass procedure is very often used for distal bypass especially in cases associated with critical limb ischemia. We experienced a case of in situ saphenous vein bypass graft aneurysm following an in situ femoro-peroneal bypass operation. An 85-year-old woman presented with a complaint of pain of the right leg at rest. CT angiography revealed superficial femoral and popliteal artery occlusion with atherosclerostic changes. In situ femoro-peroneal bypass was uneventfully performed. Soon after discharge she noticed a pulsating mass of the knee portion of the graft and it was gradually increased. Graft aneurysm was diagnosed by CT scan And 14 weeks later we performed aneurysm resection and repair by the contralateral great saphenous vein. Injury of the graft wall by a valvulotome might have been the cause. Gentle and careful manipulation is mandatory while using the valvulotome in in situ bypass procedures.