Survival rates among breast cancer patients and the number of patients living with treatment side effects have improved, leading to increased focus on quality of life (QOL). The objective of this study was to determine the efficacy of CoQ on QOL scores among breast cancer patients in Iranian undergoing tamoxifen therapy.Thirty breast cancer patients were randomized into two groups. The first group received 100 mg CoQ, and the second group took fplacebo once a day for 8 weeks. QOL was evaluated by a standard QOL questionnaire and a specific questionnaire on QOL of breast cancer patients at baseline and the end of the study. Also, physical activity of patients was assessed with the IPAQ questionnaire and dietary intake determined by a 3-day dietary record.The data of 30 subjects were analyzed. According to QOL C30 data, CoQ led to a significant increase in physical functioning (=0.029), emotional functioning (=0.031), and cognitive functioning (=0.023) compared to placebo. Symptom scales revealed a notable reduction in appetite loss in the first group (=0.01). Global health status showed no significant changes in either study arm. On the QOL BR23, progress in functions and decline in symptoms were not statistically significant. Arm symptoms showed significant reduction (=0.022) in patients that received placebo.This trial indicates that CoQ supplementation has effects in ameliorating some dimensions of QOL in breast cancer patients. To generalize the results, larger and longer intervention studies are needed.IRCT2015042021874N1.