This is a report to describe the procedure of the endoscopic endonasal skull base reconstruction using a vascularized nasoseptal flap. Between November 2008 and October 2009, 22 patients underwent reconstructions using a nasal septal flap combined with a balloon catheter (flap group) for closure of large dural defects after the endoscopic endonasal skull base approaches. The method is as follows; We start to harvest the nasal septal flap as a mucoperichondrial flap based on the posterior septal branch of sphenopalatine artery. The middle nasal turbinate is usually dissected. The first incision is the anterior vertical incision made with unipolar electrocautery at the intercutaneomucous point of the nasal vestibule. The second incision is made the along the floor of the nasal cavity from the choana to the location of the initial anterior incision. Thereafter, the superior incision to the sphenoid ostium is made 1.0∼1.5cm below the most superior aspect of the nasal septum. The pedicle of the flap formed in the width from the sphenoid ostium to the choana is extended laterally to the level of the sphenopalatine foramen. The nasal septal flap is usually placed in the nasopharynx until using in later reconstruction. On the reconstruction phase, the nasal septal flap is laid directly on the large dural and bony defect and fat grafts are applied outside with fibrin tissue glue. A Sinus balloon catheter is finally placed as support for 7∼10 days. Otorhinolaryngological endoscopic assessments are regularly performed at outpatient clinics. Postoperative CSF leaks occurred in two patients (9.1%) Conclusions: Our endoscopic endonasal skull base reconstructions using a nasal septal flap combined with a balloon catheter are useful and reliable for ventral skull base defects after endoscopic endonasal approaches.