Methane emissions from hydroelectric reservoirs can comprise a considerable portion of anthropogenic methane. However, lack of data on CH 4 emissions in different geographical regions and high spatial-temporal variability in the emission rates of reservoirs has led to uncertainties regarding regional emission estimates of CH 4 . In the subtropical plateau climate region, we used the Ertan hydroelectric reservoir as a study area. The CH 4 flux at the air-water interface was assessed by floating chambers and factors influencing emissions, including the distance from the dam, water depth, seasonal variation in wet and dry season, air-water temperature gradient and wind speed, and was also studied through a year-long systematic sampling and monitoring experiment. The results showed that the surface of the reservoir was a source of CH 4 during the sampling period and the annual average CH 4 flux was 2Ð80 š 1Ð52 mg m 2 d 1 . CH 4 flux (and its variation) was higher in the shallow water areas than in the deep-water areas. CH 4 flux near the dam was significantly higher than that of other locations farther from the dam in the dry season. The seasonal variations of CH 4 emission in wet and dry seasons were minor and significant diurnal variations were observed in wet and dry seasons. Exponential relationships between the CH 4 flux and air-water temperature gradient were found. Air-water temperature gradient was an important factor influencing diurnal variations of CH 4 flux in the Ertan hydroelectric reservoir. These results indicate that systematic sampling is needed to better estimate CH 4 flux through coverage of the spatial variation of different water depths, measuring-point distance from the dam, seasonal variation in wet and dry seasons and changes in climate factors (such as air-water temperature gradient). Our results also provide a fundamental parameter for CH 4 emission estimation of global reservoirs.