The transsphenoidal approach to the pituitary is widely used in pituitary surgery. Even though there are some landmarks for internal carotid artery (ICA) on the wall of the sphenoid sinus, it is not rare to get the artery injured during surgery. We found that the most important landmark, carotid prominence, matched with ICA in merely 37.5% of subjects. In order to find a simple method to locate the artery, we made an anatomical measurement of the ICA and placed the results in a 3-dimensional coordinate system. The sphenoid sinus opening is both the center of the endoscope entry in the anterior sinus wall and the origin of the coordinate system containing 3 orthogonal axes: x, y, and z. The x axis follows the body of the endoscope(out of the sphenoid sinus) parallel to the sagittal plane while z is perpendicular to the sagittal plane. Most of the measurements were obtained in the initial operative plane, which is perpendicular to the sagittal plane and contains the sinus opening and the midpoint of the pituitary fossa. We calculated the coordinates of the midpoint of the pituitary fossa and 4 ICA-related points. The depth of an ICA and the distance between 2 ICAs are also helpful in locating ICA. According to our operation method, all the projective points of the medial edge of ICA on the posterior wall of the sphenoid sinus are lateral to the sphenoid sinus opening, and operating within 0-25 degrees medial to the endoscope body is believed to be safe from ICA injury.