The parasympathetic nervous function in obstructive sleep apnea syndrome (OSAS) patients were quantified by using the respiratory sinus arrhythmia (RSA) impulse response magnitude. Ten participants were recruited and classified into three groups; mild (four participants), moderate (four participants) and severe (two participants) groups. Autoregressive with exogenous input (ARX) model was used to precisely separate the effect of respiration and systolic blood pressure to heart rate variability (HRV). The RSA impulse response magnitude, the parasympathetic autonomic function index, was estimated during supine and standing conditions. We found that the RSA impulse response magnitude during supine and standing, orthostatic stress, remained unchanged. Therefore, parasympathetic autonomic function impair in OSAS.Index Terms-Obstructive sleep apnea syndrome (OSAS), The RSA impulse response magnitude, parasympathetic autonomic function