A 34-year-old female patient with fibrous dysplasia of left maxilla underwent hemi-maxillectomy and reconstruction with abdominal rectus musculocutaneous free-flap. It was diagnosed that defatting of the bulky flap was needed in order to obtain space for the denture and to improve esthetics. Therefore, a surgical guide, which gave the ideal shape of the postoperative site in the maxilla, was prepared. The results of the defatting based on the surgical guide were investigated by intraoral volumetric measurement using a dental cast. The values of intraoral volume at pre-hemimaxillectomy, post-reconstruction and post-defatting with dent-maxillary prosthesis were 20.07, 15.86, and 17.24 cm³, respectively. For reconstruction with prosthesis, this procedure could be effective to maintain the intraoral space after reconstructive surgery.