Background and Aim: Oral mucositis (OM) is a common side effect of systemic chemotherapy (CT) in cancer patients. The aim was to evaluate the effect of low-level laser therapy (LLLT) for the treatment of CT-induced OM children. Patients and Methods: The study was carried out in 40 pediatric patients 3 to 18 years of age, who were hospitalized for the diagnosis of leukemia and underwent CT between June 1, 2019, and December 31, 2019. They were randomly divided into 2 groups with 20 cases in each group. The LLLT group was treated with GaAlAs diode laser (l): 830 nm (infrared), power: 150 mW, dose: 4.5 J/cm(2) and the control group underwent bicarbonate treatment. Patients received intervention for 3 days. The grade of OM was clinically assessed by the World Health Organization (WHO) Common Toxicity Criteria Scale. Visual Analog Scale was used on the same days with OM grade to evaluate the pain status. Results: While there was no significant difference between the groups in terms of OM grade at the beginning of the treatment and on the first, second, fourth, and 11th days of the treatment, the average OM grade of the LLLT group was found to be statistically significantly lower on the third, fifth, sixth, and seventh days of the treatment. The Visual Analog Scale score of the LLLT group was statistically significantly lower compared with the control group at all examinations starting from the first day of treatment (P<0.05). Conclusion: In the treatment of oral OM that occurs in children after CT, both standard care and LLLT treatment improve the grade and pain of OM.