Background: Surgery to anterior skull base is challenging since complex structures are involved. Injury of olfactory fila and optic nerve may result in postoperative complications. In our study, the authors aim to use computer topographic image to provide a comprehensive anatomical information of anterior skull base and set up a new classification of optic canal (OC) in ethmoid sinus by the degree of bony defect, so as to help surgeons in endoscopic transnasal approach to anterior skull base. Method: Computer topographic angiography images of 112 individuals were reviewed, the measurement was performed on coronal, sagittal, and axis planes after the multiplanar reformation. Nostril and mid-sagittal lines were used to locate the anterior, middle, and posterior part of ethmoidal foramina and orbital opening of OC. Further, the authors classified OC in ethmoidal sinus into 4 types by the degree of the bony defect. Result: Distance between nostril to anterior, middle, and posterior part of ethmoidal foramina and OC are 6.23 +/- 0.21, 6.62 +/- 0.26, 7.29 +/- 0.25, and 7.76 +/- 0.41 mm, respectively. Angle between line from nostril to ethmoidal foramina and horizontal plane are 47.50 degrees +/- 1.03 degrees, 41.67 degrees +/- 1.33 degrees, 37.20 degrees +/- 1.34 degrees, respectively. For the 4 types of OC, the percentage is 15.6%, 11.6%, 31.3%, and 41.5%, respectively. Conclusions: Our findings provide anatomical information of ethmoidal foramina and OC during endoscopic transnasal approach to anterior skull base, on the basis of some fixed anatomical landmarks. So as to enhance the surgical safety of this procedure and aid in the choice of the appropriate endoscopic equipment for the procedure.