Objective: To evaluate the accuracy of cone-beam computed tomography (CBCT) in the assessment of mandibular molar furcation defects. Material and Methods: Thirty patients with furcation defects were selected, oral hygiene instructions, scaling, and root planing with ultrasonic devices and hand instruments and occlusal adjustments were performed. Pre-surgical clinical measurements were carried out at the buccal aspect of the selected mandibular molars. The horizontal furcation measurements were measured with a Nabers Probe starting at the furcation entrance to the greatest horizontal depth. The degree of furcation involvement was graded from 0 to III. Bone loss in the horizontal and vertical direction and the width of the furcation entrance were measured on CBCT and after reflecting the full-thickness flap and debridement of the defects. The data were analyzed using t-test and Pearson's correlation coefficient. Results: The width of furcation entrance in clinical method was 3.27 ± 0.77, while in CBCT method was 3.35 ± 0.71, clinically the vertical bone loss was 3.61±1.09, while in CBCT was 3.57 ± 1.15, horizontal bone loss in clinical method was 5.08 ± 2.21, while in CBCT was 5.11 ± 2.23. No significant difference between the two methods was noted, and a high correlation between the two methods was observed. With regards to the agreement between the two methods of assessment, the width of furcation entrance revealed a difference between the two methods by 0.08 ± 0.21, while vertical bone loss showed difference between the two methods by -0.04 ± 0.19, the horizontal bone loss showed a mean difference between the two methods by 0.03 ± 0.21. Conclusion: CBCT provided high accuracy for the furcation involvement detection and anatomy of surrounding periodontal tissues.