The purpose of this study was to compare the effect of two extraction patterns on dental and facial vertical dimension (FVD) changes in the treatment of Class I Malocclusions of mild or moderate crowding. Pre- and posttreatment cephalometric films and dental casts of 48 Class I malocclusion patients treated by one experienced orthodontist were randomly selected from completed premolar extraction cases. One group (five males and nineteen females) involved the extraction of mandibular second premolars and the other (six males and eighteen females) with extraction of mandibular first premolars. Each group consisted of 24 subjects with maxillary first premolar extracted, treated with preadjusted edgewise appliance. To determine changes due to treatment and to compare differences between two groups, paired t-test and independent t-test were conducted, respectively. Results showed that either extraction patterns could achieve satisfactory effect, including Class I molar relationship, normal overjets and overbites. Evaluation of the treatment results of the two groups showed that no significant difference was found in the mesial movement of molars and retraction of incisors between the two groups. Increased FVD occurred in either group after extraction treatment while there was no significant difference between the two groups. In both groups, there was no significant change in the mandibular plane angle and the MM angle. This lend no evidence to the hypothesis that premolar extraction sequence was associated with mandibular counterclockwise rotation or reduction in the vertical dimension in subjects with Class I Malocclusions demanding mild or moderate anchorage.