Objective: The purpose of this study was to evaluate the effect bracket slot size on the rate of the tooth movement during leveling and alignment stage in the mandibular dental arch. Patients and methods: 20 patients (15 females, 5 males with a mean age of 16.93 ± 2.22 years in group 1 and 17.31 ± 2.65 years) in group 2 who needed fixed orthodontic treatment and extractions of their maxillary and mandibular first premolars were allocated equally into two groups randomly as follows: Group 1, who had bracket slot 0.022, while Group 2 who had bracket slot 0.018. Eligibility criteria included: Age range of 14 to 18 years, cases of malocclusion requiring extraction of the maxillary and mandibular first premolars, good general and oral health, no systemic disease, and no previous orthodontic treatment. The leveling and alignment was done in the mandibular arch after extraction of bilateral first premolars. Results: Clinical assessments showed that there was not statistically significant difference in the rate of tooth movement during leveling and alignment at T0–T6 between two groups (P ≥ 0.05). Conclusions: It was concluded that both types of brackets can be used, and both are effective in the leveling and alignment stage of orthodontic treatment, however bracket slot 0.018 showed non-significant faster rate of tooth movement than bracket slot 0.022. [