OBJECTIVE To compare the dental change and the mechanism after non-extraction treatment between passive self-ligating and conventional bracket systems. METHODS Twenty-six non-extraction crowding cases were selected. Half of them were treated with Damon3 self-ligating brackets and the others were treated with conventional brackets. The results were compared. Stepwise regression analysis was performed for the two groups. RESULTS There were no statistically significant differences in the changes of the intercanine and interpremolar widths and L1-MP angle (P > 0.1). In the self-ligating bracket group, the intermolar width increased by 1.42 mm and the L1-APo increased by 2.66 mm. In the conventional appliance group, the intermolar width increased by 0.65 mm and L1-APo increased by 1.57 mm. The differences were statistically significant between the two groups (P < 0.1). Stepwise regression analysis showed that in self-ligating bracket group, the coefficient of determination for the changes of L1-MP was 96.6%. CONCLUSIONS In crowding cases treated with non-extraction, there were an overall increase in the proclination of the mandibular incisors and the arch width in both bracket groups. Compared with cases treated with conventional appliances, the cases treated with Damon3 brackets resulted in greater intermolar width increases; In Damon3 bracket group, the change of the lower incisor inclination was influenced not only by the mandibular crowding and the intermolar width before treatment but also by the patient's skeletal pattern and the changes of the arch width during treatment.