Although there are several methods for treating fractures of long bones in lower extremity, nonunion has often developed. Cave(7) (1960) has defined nonunion as a fracture in which the reparative processes have come to a comeplete standstill. The factors of nonunion have been known as infection, extensive soft tissue injury, interposition of the soft tissue, disturbance of the blood supply, insecure fixation and distraction of the fracture site. Authors carried out retrospective study of the 36 nonunions of the long bones of the lower extremities, who were treated at the Orthopedic Department, Kang-Nam St. Mary's Hospital from July 1981 to May 1987, to find out the relationship between type of fracture and type of non-union, and the reasons of the non-union of the tibia and femur. The results are as follows : 1. Among 36 cases of nonunion, 22 cases(61%) were comminuted fractures, and 8(67%) out of 12 cases of tibial nonunion were open fractures. 2. Hypervascular nonunion was prevalent in femur ; 15 out of 24 cases of femur fractures were hypervascular, while 7 of 12 cases of tibial fracture were avascular nonunion. 3. The main causes of the femoral nonunion were the insecure fixation, failure of anatomical reduction of the medial buttress at the fracture site, and impaired blood supply of the fracture ends in orders. 4. Among all 36 cases of nonunion 20 cases were hypervascular, and 16 cases were avascular type. 12(75%) out of 16 cases of avascular nonuion were comminuted fractures. 5. All three cases of the femoral nonunion which was induced by the failure of interlocking K-nail, had fracture within 7.5cm from the interlocking hole. 6. Six cases of nonunion having large bone defect and failed nonunion after repeated bone graft procedure were successfully treated by the graft plus the total invasive type of electrical stimulator.