Abstract: Evapotranspiration (ET) is a key factor in the eco-hydrological process on basin scale. However, it is hard to obtain long-term and reliable ET measurement data for a specific watershed. In this study, ET reanalysis products from the European Centre for Medium Range Weather Forecasts (ECMWF) and the U.S. National Centers for Environmental Prediction (NCEP) were compared to eddy covariance measurements in the Yellow River source region on daily scale. The ERA-Interim reanalysis data was found to have a good agreement with ET measurements in the Yellow River source region with the root mean square error (RMSQ) of 0.63; the NCEP reanalysis data was found to overestimate ET from April to July and from October to December with the RMSQ of 0.81. The Mann Kendall test and Sen's slope estimator test were then applied to quantify the changing trend of ET during 1979-2014 based on the Interim-ET reanalysis products. In general ET increased over the northern region and decreased in the south during the study period. The most rapid increase of 1.5-2.5 mm/a was found in the northern region in Xinhai-Gonghe-Guide, and the most rapid decrease of -1.0--0.5 mm/a was found in the southwestern region in Qumalai-Zhidu-Yushu. In the southeastern region in Maqin-Maqu-Jiuzhi, ET also increased by 0.5-1.0 mm/a. Abrupt ET changes over Yellow River source region mainly occurred in the 1980s based on results of both the moving t test and the sequential Mann Kendall test.