This study was designed to prove that survival of flaps was improved by the hypobaric oxygen training before making flaps. Wister strain male rats weighing 200-250 grams were used as the experimental animals. Rats were kept in a hypobaric simulator two hours a day for two weeks, which was set to emulate a condition of 8000 meters above sea level (hypobaric oxygen training). When the hypobaric oxygen training was finished, flaps were elevated on the back of the rats. Survival rate of flaps were significantly increased at 7th post-operative day. But its effect has been diminished until the 3rd training day. The mechanism of hypobaric oxygen training was studied by blood cell count and measurement of serum prostglandin E level, micro-angiogram of the flap and the count of insulin particle in the dermis. Consequently, it is postulated that the mechanism of hypobaric oxygen training was not morphological and functional change in the flap vascularity but the cellular metabolic adaptation to the hypobaric state.