Abstract: Object To evaluate the maxillomandibular transverse basal discrepancies and dental arch discrepancies in the first molar area, and to compare values between skeletal Class III malocclusion and Class I normal occlusion groups. Methods CBCT scans of 30 patients with skeletal Class III malocclusion and 30 with Class I normal occlusion were evaluated. Basal bone width were measured as the distance between zygomatic arch point for the maxilla and for the mandible measured between the most buccal point on the cortical plate opposite the mandible first molars at the level of the center of resistance. Dental arch width were measured as the distance between the right and left fovea of the first molars using dental casts. Independent-sample t tests were used with SPSS(version 22). Results by comparing the skeletal Class III groups and the Class I normal occlusion groups ,the maxillary basal bone width, the mandibular basal bone width, the maxillomandibular basal bone discrepancies showed significant difference (P<0.05) with skeletal Class III groups (63.96±3.78mm) smaller than Class I normal occlusion groups （65.67±2.76mm）, skeletal Class III groups (62.26±3.12mm) bigger than Class I normal occlusion groups （60.29±3.15mm）, skeletal Class III groups (2.31±2.41mm) smaller than Class I normal occlusion groups（5.38±1.24mm）respectively. In the contrast, there was no significant difference in the dental arch width (P>0.05). Conclusion Skeletal Class III malocclusion groups tend to show maxillomandibular transverse basal bone disharmony and compensatory inclination.

Key words: skeletal Class III malocclusion, basal width, dental arch width, CBCT