Methane emissions from hydroelectric reservoirs can comprise a considerable portion of anthropogenic methane. However, lack of data on CH4 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 CH4. In the subtropical plateau climate region, we used the Ertan hydroelectric reservoir as a study area. The CH4 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 CH4 during the sampling period and the annual average CH4 flux was 2·80 ± 1·52 mg m−2 d−1. CH4 flux (and its variation) was higher in the shallow water areas than in the deep-water areas. CH4 flux near the dam was significantly higher than that of other locations farther from the dam in the dry season. The seasonal variations of CH4 emission in wet and dry seasons were minor and significant diurnal variations were observed in wet and dry seasons. Exponential relationships between the CH4 flux and air-water temperature gradient were found. Air-water temperature gradient was an important factor influencing diurnal variations of CH4 flux in the Ertan hydroelectric reservoir. These results indicate that systematic sampling is needed to better estimate CH4 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 CH4 emission estimation of global reservoirs. Copyright © 2010 John Wiley & Sons, Ltd.