Background: The aim of this study was to investigate the relationship between thickness of masseter muscle and mandibular morphology of dentulous and partially posterior edentulous patients. Methods: 90 female patients between ages of 40 and 60 were divided into two groups: control dentulous group and posterior edentulous group. The measurements of control group and bilateral posterior edentulous patients were performed bilaterally and the averages of right and left were obtained. Only the toothless side of unilateral edentulous patients was measured. Ultrasonography was used to examine the thickness of masseter muscle, during rest and maximum voluntary contraction, twice with five minutes intervals by the same observer. Gonial and antegonial angles were analysed on panoramic radiography to evaluate mandibular morphology, twice with one week interval by the same observer. Intraclass correlation coefficient test, Kolmogorov-Smirnov test, Levene test, independent sample t-test and Pearson correlation test were used in the statistical analysis of data. Results: There was no statistically significant association between masseter muscle thickness and posterior partially edentulism (p˃0.05). The results of the Pearson’s correlation test showed that masseter muscle thickness in both control and experimental groups was negative correlated with the gonial angle (-0.402≤r≤-0.276). Conclusion: Posterior partially edentulism does not reason a significant decrease of masseter muscle thickness (p˃0.05). The functional activity of masseter muscle is influential in the formation of the gonial region. Ultrasonography is found to be a reproducible, reliable and accurate tool to examine the thickness of masseter muscle.