Factor XI deficiency (also called Hemophilia C) rarely occurs among ethnicities other than Ashkenazi Jews. A boy was scheduled for frontoethmoidectomy due to bilateral chronic rhinosinusitis. He was incidentally found to have factor XI deficiency due to prolonged aPTT on preoperative laboratory finding. His medical history reveals frequent epistaxis 2 or 3 times per day and his factor XI and XII activity were 17% (normal; 60-140%) and 34% (normal; 60-140%), respectively on furthermore laboratory evaluation. He was diagnosed as hereditary factor XI deficiency. He underwent the operation with administration of the fresh frozen plasma without complication.