Aim: Is to compare the efficiency of passive self-ligating versus conventional brackets in the alignment of crowded mandibular teeth. Materials and methods: A randomized clinical trial with Fifty participants visiting the clinic of the Orthodontic Department and fulfilling the eligibility criteria were enrolled in the study. Patients were randomly allocated to group A (Conventional brackets) or group B (Damon Q metal passive self-ligating brackets). The amount of mandibular crowding was assessed by Little's irregularity index. Subjects in both groups were recalled for follow-up every four weeks till completion of alignment. Results: There was no significant difference in baseline irregularity in both groups. The time needed in months to achieve zero Little irregularity index was insignificantly shorter in group B (5.04 ± 1.30) than in group A (5.05 ± 1.20) (P=0.99). Number of visits in group A (5.76± 1.30) was insignificantly lower than group B (5.87 ± 1.69) (P=0.86). Correlation between the initial little irregularity index and rate of improvement revealed a significant positive correlation between them in both groups. The rate of improvement was (0.96 ± 0.09) with little irregularity index 4 and increased to (1.17 ± 0.24) with little irregularity index 8. Conclusions: There was no time difference between passive self-ligating brackets and conventional brackets in correcting mandibular crowding. Furthermore, no difference in the number of visits. The amount of initial irregularity and the timing were the factors that affected the rate of alignment.