Objective:To investigate the relationship between obstructive sleep apnea syndrome(OSAS) and early atherosclerosis using vascular ultrasonography. Methods:Thirty-seven OSAS patients without hypertension (OSAS group) and forty normal subjects were enrolled in the study. Flow-mediated dilation(FMD) was investigated with brachial artery ultrasound examination. In addition,the arterial elastic parameters including vascular expansibility(VE),compliance coefficient(CC),stiffness index(-茁) and pulse wave velocity (PWV) through quantitative arterial stiffness (QAS) analysis were measured. The differences of the parameters between two groups were analyzed. In the OSAS group,polysomnograph(PSG) data were recorded and analyzed including apnea-hypopnea index (AHI),minimal pulse oxyhemoglobin saturation(miniSpO2). Correlations and regression were calculated between indices of oxygen saturation and FMD. Twenty patients in OSAS group underwent continuous positive airway pressure(CPAP) treatment for three months. Results:① Compared with normal control group,FMD,VE and CC in OSAS group were significantly lower,but -茁 and PWV were significantly higher(P < 0.05). ② Through pearson correlation analysis,in the OSAS group,FMD was negatively correlated with AHI,age and -茁(r = -0.325~-0.378,all P < 0.05),but positively correlated with miniSpO2(r = 0.504,P < 0.01). Through stepwise multiple linear regression analysis,miniSpO2 was a significant determinant for FMD (R2 = 0.548,P < 0.05). ③ Compared with parameters before treatment,there was a significant increase in FMD and CC,but the parameters of arterial stiffness,including PWV and -茁,were significantly lower(P < 0.05). Conclusion:Endothelial dysfunction and the arterial elasticity decrease are present in OSAS patients. Vascular ultrasonography could be performed in detection of early atherosclerosis. CPAP treatment can improve vascular elasticity and endothelium function effectively.