The objective of this study is to evaluate the potential utility of the USGS Global Data Assimilation System (GDAS) 1-degree, daily reference Evapotranspiration (ET(0)) products by comparing them with observed Oklahoma mesonet daily ET(0) over a 2 year period (2005-2006). The comparison showed a close match between the two independent ET(0) products, with bias within a range of 10% for most of the sites and the overall bias of -aEuro parts per thousand 2.80%. The temporal patterns are strongly correlated, with a correlation coefficient above 0.9 for all groups. In summary, we conclude that (1) the consistent low bias shows the original GDAS ET(0) products have high potentials to be used in land surface modeling; (2) the high temporal correlations demonstrate the capability of GDAS ET(0) to represent the major atmospheric processes that control the daily variation of surface hydrology; (3) The temporal and spatial correspondences in trend between independent datasets (GDAS and MESONET) were good. The finding in Oklahoma, a different hydro-climate region from a similar regional study conducted in California by Senay et al. (J Am Water Res Assoc 44(4):969-979, 2008), reconfirms the reliability and potential of using GDAS reference ET for regional energy balance and water resources management in many parts of the world.