Aims: To evaluate the effects of obstructive sleep apnea (OSA) on the left atrial (LA) function and its associations with left ventricular (LV) diastolic function independent of obesity. Methods and Results: In 49 subjects (24 non-OSA and 25 OSA, body mass index ��� 25 kg/m2), standard and tissue Doppler imaging (TDI) based strain echocardiography was performed. There were no significant differences in the LV dimension, LV wall thickness, LV ejection fraction (EF), early mitral diastolic velocity (E), LA dimension, LA EF, and LA volume index. Late mitral diastolic velocity (A), however, was significantly augmented in the OSA group (92.4짹9.3 cm/sec vs. 75.3짹13.1, P <0.001) and mitral annular early diastolic velocity (Ea) was vice versa (6.4짹0.4 vs. 7.3짹0.2 cm/sec P <0.001). OSA patients had significantly higher diastolic filling pressure (E/Ea, 9.09짹4.3 vs. 10.8짹2.8, P=0.04). The mean peak systolic strain rate (SmSR) and mean peak late diastolic strain rate (AmSR) had not shown differences in both groups. The mean peak systolic strain (Sm, 55.1짹13.6 vs. 43.2짹10.1 %, P <0.001) and mean peak early diastolic strain rate (EmSR, -4.08짹0.29 vs. -2.10짹0.85 sec-1, P <0.001) of the OSA group were significantly lower. Apnea-hypopnea index (AHI) of the OSA patients had significant correlations with E/Ea (r=0.67, P <0.001). AHI and E/Ea had significant effects on LAVI, Sm, SmSR, and EmSR of LA. Conclusion: OSA impaired LA wall compliance and passive contraction independent of obesity. LA structural and functional remodeling was significantly correlated with the severity of OSA and LV diastolic filling pressure.