Objective: The aim of this study was to determine palatal and maxillary arch width changes during MBT PEA with maxillary first premolars extraction treatment in patients with Class II division 1 malocclusion. Material and Methods: Dental casts of 24 Class II division 1 patients (8 males and 16 females) were evaluated. The minimum age of the subjects at the beginning of treatment was above 16 years with mean age 02.02± 2.82. All patients were treated with extraction of the maxillary first premolars and MBT PEA. Pre- and post-treatment maxillary and palatal (inter-canine and inter-molar) widths were measured using a digital calliper of 0.01 accuracy. Paired samples t-test was used to evaluate the treatment changes within total sample and within males group and females group. Differences between males and females were tested using independent samples t-test. Results: At the end of treatment, maxillary and palatal inter-canine widths increased significantly (P>0.001).The maxillary inter-molar width increased but the values were statistically insignificant (P<0.05). The palatal inter-molar width increased significantly (P>0.01). No significant differences were observed between males and females. Conclusions: Class II division 1 patients treated with extraction of maxillary first premolars had tendency for an increase in palatal and arch width during treatment except maxillary inter-molar width. It seems that a more thorough assessment and investigation including pretreatment arch shape and factors related to anchorage should be carried out.