Abstract : This study aimed to compare the dentoalveolar changes following the orthodontic treatment with clear aligners in comparison with the traditional fixed appliances. A single-center, 2-arm parallel-group RCT was conducted. Thirty-six adult patients (20 females, 16 males) who had severe crowding and required first-premolar extraction were recruited and allocated to the clear aligners group (CA), or the fixed appliances group (FA). Pre-and post-treatment dental cast models were evaluated by measuring the intercanine, interpremolar, and intermolar widths, arch length, and arch depth. Wilcoxon signed-ranks tests were used for intra-group comparisons, whereas Mann-Whitney U tests were used for inter-group comparisons. Bonferroni's adjustment of the alpha level was adopted. In the CA group, there were no significant differences in the maxillary and mandibular intercanine and interpremolar widths, whereas there were significant differences in the mandibular interpremolar width, maxillary and mandibular arch length, and depth (P≤0.001). In the FA group, there was a minor decrease in the transverse maxillary and mandibular arch widths with no significant differences. There was a significant decrease after treatment with the fixed appliances for the arch length and depth. There were no significant differences in the transverse arch widths, arch lengths, and arch depths (P>0.005) between the two groups. The orthodontic treatment with four first-premolar extractions of severe crowding patients by the clear aligners or the fixed appliances is accompanied by slight and insignificant transverse changes on the arches, except for the interpremolar width in the lower premolar area in the CA group.