Objective : There have been few comprehensive studies on the analysis of 24-hour HRV of major depressive disorder (MDD). The purpose of this study was to compare the autonomic nerve system of patients with a MDD with healthy patients and to examine the physiologic and clinical effects of 24-hour HRV by analyzing whether the HRV demonstrates the level of depressive symptoms after improving the symptoms in patients with a MDD. Methods : The 24-hour HRV was measured in patient groups with a MDD (n=16) and control groups (n=16). The patients with a MDD received the follow up test for two months after the treatment. Results : There were significant differences among the indexes (SDNN, rMSSD, SDNN index, and pNN50) of time-domain analysis and the indexes (TP, VLF, LF, HF, and ULF) of frequency-domain analysis of HRV between patient and control groups. The means of RR, SDNN, SDANN, and TP increased after two month of the treatment, comparing them with before the treatment, but there were no statistical significance. Conclusion : The results of 24-hour HRV analysis indicated significant decrease of HRV indexes among MDD patients which may suggest decrease of parasympathetic nervous functions.