A posterior tympanotomy is a procedure performed to access the posterior mesotympanum to remove pathologic lesions through a mastoid exposure. In this study, we aimed to evaluate and classify the anatomical relationships between the vertical segment of the facial nerve and the mastoid antrum in an axial temporal bone CT to prevent facial nerve injury when performing a posterior tympanotomy. We performed retrospective analysis of temporal bone CT scans. A cohort of 102 patients who underwent tympanomastoidectomy for chronic otitis media, and 43 patients with sensorineural hearing loss with normal temporal bone, were included. All patients had preoperative and diagnostic computed tomography of the temporal bone from January to December 2006. Anatomical relationships were evaluated according to several parameters: protrusion of the facial nerve into the mastoid antrum, status of the facial canal, new bone formation over the facial canal, and size of the mastoid antrum. Anatomical relationships between the vertical segment and the mastoid antrum were classified into six types. Type 1 has no protrusion of the facial nerve with a regular facial canal, and Type 2 has no protrusion with a partly thin facial canal. Type 3 has protrusion into the antrum with a regular facial canal, and Type 4 has protrusion with a partly thin facial canal. Type 5 has new bone formation around the facial canal, and Type 6 has new bone formation with a small or contracted antrum. These results may enable preoperative assessment of the facial nerve's status, and provide background knowledge to prevent facial nerve injury when performing a posterior tympanotomy.