Facial nerve edema is an important finding in Bell's palsy patients. Inflammation may cause facial nerve edema, and mechanical compression and ischemic change of the facial nerve may occur in the facial nerve canal. A few studies have reported the dimensions of the facial nerve canal using conventional computed tomography or human temporal bone sections. However, the cross-sectional area of the facial nerve canal has not been fully understood. Therefore, the cross-sectional area of the facial nerve canal was measured in patients with unilateral Bell's palsy by computer tomography with multiplanar reconstruction. Sixteen patients with unilateral Bell's palsy were enrolled. Computed tomography of the temporal bone was performed, and perpendicular images to the facial nerve canal were reconstructed by the multiplanar reconstruction technique. The cross-sectional area of the facial nerve canal on the affected and unaffected sides was measured at the labyrinthine segment, the horizontal segment, and the mastoid segment. Both in the labyrinthine and horizontal segments, the mean cross-sectional area of the facial nerve canal was significantly smaller on the affected side than on the unaffected side. There was no significant difference between the affected and unaffected sides in the cross-sectional area of the facial nerve canal in the mastoid segment. The labyrinthine segment was the narrowest segment in the facial nerve canal. These findings suggest that the facial nerve is vulnerable, especially in the labyrinthine segment of the facial nerve canal, and a narrow facial nerve canal may be one of the risk factors for Bell's palsy.