For a patient with fourth-degree burns involving both legs, we performed an amputation of the right leg at the thigh and covered the left knee joint with the posterior part of the right leg as a cross-leg fillet flap. It is reasonable to use tissue from one leg for reconstruction of the other leg in order to avoid bilateral leg amputation in the treatment of bilateral severe leg injury. We could transfer a sufficient amount of tissue by using this flap without donor site morbidity. Generally, hip or knee joint contracture is a major problem with the cross-leg flap. The long pedicle of this flap allowed the patient some leg movement and there was no contracture at his knee or hip joints. We cannot use this flap as a sensory flap because of pedicle cutting, but inappropriate sensation of the lower extremity is uncomfortable and reduces a patient's quality of life. The cross-leg fillet flap described here avoids donor site morbidity without the use of microsurgery. This flap will become a treatment option for severe bilateral leg injuries when one leg requires amputation.