Statement of Problem: There must be a proper mesiodistal tooth size ratio (Bolton analysis) between maxillary and mandibular teeth for good occlusal interdigitation. Therefore the Bolton analysis should be considered during diagnosis, treatment planning and predication of ultimate results.Purpose: The purpose of this study was to appraise tooth size ratios in Cl II malocclusion group and compare them with normal individuals.Materials and Method: This study was carried out on 60 pre-treatment orthodontic casts of class II malocclusion patients and 60 diagnostic casts of normal occlusion individuals which were selected through cluster sampling in accordance with the selective criteria. Each group consisted of 30 men and 30 women. The greatest mesiodistal diameters of all the teeth on each cast were measured by a digital calliper with 0.01mm accuracy except the second and third molars. Then tooth size ratios were analyzed as Bolton described. The statistical analysis were performed by chi-square and t-tests using SPSS.Results: The prevalence of anterior and overall tooth size discrepancy was relatively high (28.3%, 20%), showing no significant difference between men and women (p> 0.05). The mean of anterior and overall tooth- size ratios in Cl II malocclusion group were 79.18 and 92.39 respectively, which were statistically different from the Bolton study (ideal occlusion) ratios (p< 0.05).There were no statistical difference between the means of anterior and overall ratios of men and women, neither in Cl II malocclusion group nor in the normal individual group (p> 0.05).Conclusion: Considering the high frequency of tooth size discrepancy among CLII patients and the significant difference in Bolton ratios between this malocclusion and ideal occlusions; it seems that tooth size discrepancy can be considered as a possible etiologic factor and Bolton analysis should be performed as a pre-treatment diagnostic tool for this type of malocclusion.