Coagulation factor Ⅺ deficiency (also called Hemophilia C) rarely occurs other than Ashkenazi Jews,which is a rare coagulation disorder associated with prolonged activated partial thromboplastin time(aPTT).A female was scheduled for orthognathic surgery due to mandibular protrusion deformity.She was incidentally found to have factor Ⅺ deficiency due to prolonged aPTT on preoperative laboratory finding.Her and her family medical history reveals no abnormal bleeding tendency and her factor Ⅺ and Ⅻ activity were 39% (normal: 50-150%) and 51% (normal:50-150%) respectively on furthermore laboratory evaluation.She was diagnosed as factor Ⅺ deficiency, and underwent the orthognathic surgery without plasma replacement,but without complications.