Cubitus valgus may arise as the sequele of a number of different condition for example, a premature epiphysiodesis of the lateral condylar physis, nonunion or malunion of the lateral condylar fracture, rarely supracondylar fracture of the humerus. Cubitus valgus has more functional loss of extension and possibly the late development of a tardy ulnar nerve paralysis and less significant cosmetic effect. Authors treated the 7 patients who had cubitus valgus developing as a sequele of nonunion of the lateral condyle of the humerus, by the medial closed wedge osteotomy and internal fixation with the lateral arm cut anatomical Y-plate through posterior approach. Five males and two females were followed average 19 months(range 5 to 37 months). A mean age at the time of the operation was 17.1 years(range 13 to 25 years). Symptoms of tardy ulnar nerve palsy was shown in 5 patients. Two patients were treated due to disappearing cosmetic problem. The carrying angle was mean valgus 31 (range 22~50) preoperatively. The carrying angle was changed from mean valgus 9.6(range 3-15) immediate postoperatively to valgus 9(range 3-14) followed state. The duration of external immobilization after operation was mean 4.1 weeks(range 3-5 weeks). Range of motion of the elbow was fully recovered in 6 cases. The neurologic deficit of the tardy ulnar nerve palsy was fully recovered in all five patients. The result of supracondlar osteotomy was excellent in 4 cases(57.1%), good in 2 cases(28.6%) and poor in 1 case(14.3%). In conlusion, internal fixation with the lateral arm cut anatomical Y-plate after medial closed wedge osteotomy through the posterior approach can be recommanded as a method of treatment for the cubitus valgus.