The incidence of malunion in the long bone with has been reduced because of the advancements in surgical technique. However, nonunion or malunion are still observed in mechanical axis deformation of the lower limb, resulting in the overload of cartilage and instability of the joint, requiring surgical correction. Preoperative planning for malunion is very important, and accurate evaluation of the deformity is essential. Herein, we describe the indications of corrective osteotomy, choice of patients, and various surgical methods for the treatment of malunion of the long bone.