The change of measurements of impulse oscillometry (IOS) in obstructive sleep apnea syndrome (OSAS) patients and its mechanism were observed. The respiratory impedance was measured by using IOS technique and polysomnography (PSG) was monitored synchronously in 36 OSAS patients, 14 patients with chronic obstructive pulmonary disease (COPD) and 12 normal controls. Results showed that R20 in OSAS group was significantly higher than in COPD group and control group (P < 0.01). R5-R20 in OSAS group was lower than that in COPD group, but significantly higher than that in control group (P < 0.01). The levels of R20 and R5-R20 were positively correlated with severity degree of the disease. In addition, apnea-hyponea index (AHI) was positively correlated with R5 and R20 with the correlation index (r)being 0.66 and 0.86 respectively. The lowest SO2 was negatively correlated with R5 and R5-R20, with r being -0.66 and -0.79 respectively. The mean SO2 was negatively correlated with R5 and R5-R20 with r being -0.81 and -0.69 respectively. IOS technique could be used as a valuable tool for assessing the degree of upper airway obstruction in the patients with OSAS, and could help to explore its pathological mechanism.