AIM This study was conducted to assess the vertical and horizontal relationship between the maxillary sinus floor and maxillary posterior teeth roots using cone beam computed tomography. MATERIALS AND METHODS In this cross-sectional study, the CBCT images presented in the radiology department of Hamadan dental school was used. From 450 observed CBCT, 340 of them did not have the inclusion criteria of the study and ultimately 110 CBCT were selected. Totally 214 first premolars, 217 second premolars, 220 first molars and 220 second molars were included. For evaluating the relationship between the maxillary sinus floor and the posterior maxillary teeth, the classification implemented in the study of Jung in 2009 was used. The relation was evaluated by two radiologists twice with a 2-week time interval. RESULTS One hundred and ten patients with a mean age of 31.06 ± 9.7 years were assessed. Type 0 relationship (the maxillary sinus floor is located above the root tip) was most frequently observed with the first and second premolars. The most common type observed in the first and second molars was type 3 (apical protrusion is seen over the maxillary sinus floor). The relation between different types and gender was statistically meaningful, while no statistically significant difference was found between the right and left side assessments. CONCLUSION This study showed that although most of the teeth did not have contact with the sinus floor, but the more posterior the maxillary teeth, the more probability for root protruding into the maxillary sinus. It also confirmed that protrusion of teeth roots into the maxillary sinus is more common in male than female. CLINICAL SIGNIFICANCE Knowing the anatomical relation between the posterior teeth and the maxillary sinus, help the clinician in preoperative treatment planning of the posterior maxillary teeth and avoid problems that may occur during dental practice.