Statement of Problem: Tooth size discrepancies (Bolton ratio) affect the best possible esthetic and functional results at the end of treatment. The extraction decision is the most critical decision when planning treatment, and the premolars are the most common teeth in this decision.Purpose: The purpose of this study was to compare the effect of different patterns of premolar extraction on tooth size discrepancy at the end of orthodontic treatments.Materials and Method: This cross-sectional study was carried out on 200 study models of the patients (145 females and 55 males) who were selected through available selective sampling method. The greatest mesiodistal width of all teeth were measured except for the second and third molars. The measuring was done by a digital caliper with 0.01mm accuracy. According to calculated Bolton overall ratios, the patients’ casts were divided into three groups: NB (Normal Bolton), DB (Decreased Bolton), and IB (Increased Bolton). This classification was based on the overall Bolton normal values plus or minus one standard deviation. The Bolton overall ratios of each group were measured before and after hypothetical extractions of different four premolars. The data was analyzed using one way ANOVA, and the paired t-test was used for comparing the ratios between males and females.Results: The results showed that there was no significant difference between males and females in Bolton overall ratios. Significant differences were found between overall ratios for different combinations of premolar extraction before and after extractions. The Bolton overall ratios showed to be smaller after premolar extractions in each group. After premolar extractions, some patients in the NB and IB groups were transferred to the DB, and NB group respectively.Conclusion: According to the results of this study four premolars extraction causes a decrease in Bolton overall ratio and in some patients, increased overall ratios could be corrected to normal values after removal of second mandibular premolars.Key words: Bolton ratio, Tooth extraction, First premolar, Second premolar, Various malocclusion