Pulmonary hypertension (PH) is one of the major complications of obstructive sleep apnea syndrome (OSAS). Pulmonary arterial stiffness (PAS) can be used in determination of PH. The aim of the present study was to evaluate the PAS and cardiac function of patients with OSAS and analyses the relationship between OSAS severity and PAS. Sixty newly diagnosed patients with OSAS (mean age 49.6 ± 11.7 years) and 30 healthy controls (mean age 46.4 ± 14 years) were enrolled. Right ventricle (RV) and left ventricle (LV) echocardiographic parameters and PAS values of study groups were compared. There were no significant differences in terms of LV ejection fraction, LV Tei-index and tricuspid annular plane systolic excursion. PAS, mean pulmonary arterial pressure (PAP) and RV Tei-index were significantly higher but tricuspid annulus early diastolic myocardial velocity was lower in patients with OSAS than control subjects (respectively p < 0.001, p < 0.001, p = 0.001, p = 0.001). Moreover, we found a higher PAS in OSAS patients without PH compared to controls (p < 0.001). When we investigated the relationship between polysomnographic variables and echocardiographic parameters, we found positive correlations between apnea hypopnea index and total oxygen desaturation with PAS and mean PAP (r = 0.384, p < 0.001; r = 0.404, p < 0.001; r = 0.36, p < 0.001; r = 0.349, p = 0.001 respectively). PAS and mean PAP were increased in patients with OSAS. Pulmonary vascular bed may be affected due to the fluctuation of PAP during day and night time. Therefore, assessment of PAS can be more useful than PAP in OSAS patients.