Background: Altered heart rate variability (HRV) has been associated with a number of disorders affecting autonomic tone, including recent myocardial infarction, congestive heart failure, and diabetic neuropathy. Furthermore, obstructive sleep apnea (OSA) has been shown to be associated with characteristic disturbances in heart rhythm. In this study, using HRV frequency analysis, an attempt has been made to diagnose or possibly diagnose OSA. Materials and Methods: Using Somnologica version 3.3.1 software (Medcare-Embla), polysomnographic recordings were done. Electrocardiographic signals were digitalized with a sampling rate of 250 Hz. Using the HRV analysis report of this software, low-frequency (LF) and high- frequency (HF) information and LF to HF ratio (LF/HF) were obtained at 5-min intervals, then at cutting points 30, 35, 40, 45, and 50, which indicate the intensity of the apnea and hypopnea index (AHI), were analyzed with mean and standard deviation of HRV frequencies. Results: According to the results reported in this study, comparison of mild, moderate, and severe cases led to no significant differences, while frequency-domain analysis displayed significant LF/HF increase in more severe AHI cases. This can probably be applied in screening high-risk patients, reducing the application of PSG in high probable cases, and providing an estimate of prognosis of potentials patients for the physicians. Conclusions: In the study of frequency-domain analysis, LF/HF increases in more severe AHI cases. These can probably be applied in screening high-risk patients, reducing the application of PSG in high probable cases.