Background: Melanin, secreted hormone from the pituitary gland, has anti-oxidative activity and beneficial effects on sleep disorders, depression, cancer treatment and it is a cardioprotective hormone. In this study we investigated its effect on left ventricular function in patients with congestive heart failure.Methods: This study was a clinical trial among 60 patients with heart failure admitted to the training centers of Shahid Chamran and Noor hospitals of Isfahan, Iran. The studied patients were divided randomly on two groups: melatonin and control group. The patients in melatonin group received 3 mg melatonin daily and the control group received placebo for 2 months. The changes in functional class (FC) and left ventricular ejection fraction (LVEF) were cpmpared between 2 groups. Data was analyzed by SPSS version 18.and Paired t-test, Student t-test, Repeated-measures ANOVA and χ2.Finding: In the control group 60% of patients were male and 40% were female and in melatonin group 77.3% were male and the others were female. Mean age in control group was 67.1 ± 11.8 years and in melatonin group was 63.5 ± 6.7 years (P = 0.24). The LVEF in the control group has no significant change before and after the intervention (P = 0.24). But a significant difference on LVEF was seen in the melatonin group (P < 0.001).Conclusion: If the further studies approve our results melatonin can be used in treatment of heart failure along with other drugs.