We usually accept 1 : 1 ratio soft tissue change in mandibular set-back surgery. But we cannot sure whether we can use this ratio as a long term predictor after surgery. We investigated the change of hard tissue and its effects to soft tissue and examined the predictability of cephalometric analysis in short term and long term follow-up periods in mandibular set-back surgery. Subjects were 15 patients (5 male, 10 female) performed mandibular set-back procedure only by BSSRO with rigid fixation. Cephalometric data were obtained before orthodontic treatment, immediate before surgery and immediate after surgery, 6 months, and 18 months after surgery consecutively. Differences in soft and hard tissue changes among the time intervals were examined using analysis of variance (ANOVA) ; the association between immediate surgical change in chin landmarks and subsequent short and long term soft and hard tissue changes were examined using linear regression analysis 1. Soft tissue mandibular structures were positioned posteriorly and superiorly after surgery. Average mandibular set-back were 8.17mm at Pog. The horizontal and vertical hard tissue mandibular changes were stable for 18 months after surgery. Although there was a small degree of change, soft tissue and hard tissue remain relatively stable after surgery and there was no clinically discernable changes between 6 months and 18 months post-operatively. 2. Some part of upper lip (Ls & stms)and most part of mandibular soft tissue moves postero-superiorly after mandibular set-back. The ratio of horizontal changes of hard tissue to soft tissue at Inf. labial sulcus(B'), Pogonion(Pog') were 84.7%, 74.7% after 18 months. respectively. 3. Predictability of the soft tissue change is less certain than hard tissue and this predictability is decreased over time. These result suggests that we cannot predict the surgical outcomes exactly. It is recommendable that the oral surgeon should be careful in using the computerized surgical prediction software program as a predictor of long term soft tissue change.