Primary venous aneurysm, especially in pediatric population, is a very rare clinical entity. We report a case of primary great saphenous vein aneurysm in a 4-year-old boy. He was initially suspected of suffering from inguinal hernia because the soft mass was detected at the inguinal region when the patient was in the standing position, but color Doppler ultrasonography demonstrated the swelling to be a great saphenous vein aneurysm. We decided that surgery was the best option because of potential risk for thromboembolism.