Abstract: Objective: To evaluate the topographic relationship between maxillary sinus and upper molar root apices by Cone-beam Computed Tomography (CBCT) and to provide evidence for dental treatments that would get the upper molars involved. Methods: Included in this study were 98 sides of maxilla, left or right, in 53 non-sympotomatic subjects. The vertical distances from the root apices of maxillary molars to the floors of maxillary sinus were assessed by CBCT. The mean vertical distances were analyzed with interval estimation. Results: 98 sides could be classified into 4 categories according to the relationships between maxillary sinus and upper molar root apices. The distance for buccodistal root apex of first maxillary molar, the shortest one, was 0.44±0.61mm (confidence coefficient 95%). It was frequent that maxillary molars, particularly the second upper molar were close to the sinus floors. Conclusion: Maxillary molars were close to the maxillary sinus floor. The relationship could be and should be evaluated by CBCT before treatment when upper molars were involved so as to prevent an iatrogenic injury and an subsequent infection in the maxillary sinus.