This 27 year-old male patient had severe mandibular prognathism (right and left mesio-occlusion were 0.4㎝ and 1.2㎝ , respectively). He had good oral health relatively exception of missing teeth. We examined all of his oral and skeletal status with full mouth x-ray taking, study model, and cephalogram. His general condition was good but above examination indicated the surgical operation for the mandibular prognathism. His laboratory tests were within normal limits. We determined surgical operation which was done by extraoral approach bilaterally. Incisions were made bilaterally 1.5㎝ beneath the inferior border of the mandible in the selected area and then the inferior border of the ascending rami was exposed. Retracting the periosteum to the lingual and buccal a slight amount, the cut in the bone was performed by use of bone drill. Avoiding T.M.J. troubles, the proximal segmant was not fixed to anterior segment, being in overlapping state buccally, in order to expect a natural healing by the environmental muscles and ligaments. We had immobilization with intermaxillary fixation by using the multiple Stout's method. He was discharged 17 days after operation. His general condition and operation results were good and satisfactory.