Abstract: Objective To study the anatomy of the sphenoid sinus for safe pituitary adenoma resection via transsphenoidal approach through a single nostril. Methods Eight fresh adult cadavers and the magnetic resonance images (MRI) of the sphenoidal sinus of 5 adults were observed.In anteroposterior axes,the root of the nasal columella was defined as the point O,the inferior border of the sphenoidal sinus anterior wall as point B,the superior border of the spheniodal sinus anterior wall as point C,and the midpoint of the sellar floor as point D.Line OA ran through the point O in parallel with the maxillary alveolar process.∠AOB’、∠AOC’ and ∠AOD’ were the supplementary angles of ∠AOB,∠AOC and ∠AOD respectively,and ∠AOB’,∠AOC’,∠AOD’,OB,and BC were measured. Results In the 8 fresh adult cadavers,∠AOB’,∠AOC’,∠AOD’,OB,and BC were (43.2±4.3)°,(22.9±3.0)°,(35.4±4.1)°,66.3±3.6 mm,and 20.9±1.5 mm,respectively,with 2 cases having ethmoid cell superior to the sphenoid bone;in the MRI of 50 adults,the measurements were (44.1±5.5)°,(25.7±6.4)°,(34.2±5.9)°,68.7±4.9 mm,and 23.3±3.1 mm,respectively,with 15 cases having ethmoid cell superior to the sphenoid bone.Two independent-sample t test and Chi-square test revealed no significant differences in the measurements between the two groups (P>0.05). Conclusion Preoperative MRI facilitates safe opening of the anterior wall of the spheniodal sinus and the sellar floor,and the best angle between the axis of the speculum and line OA was 39°.