We have developed a novel technique that links vegetation properties and ET fluxes with a microwave “emissivity difference vegetation index” (EDVI), defined as the microwave land surface emissivity differences between two wavelengths. These EDVI values can be derived from a combination of satellite microwave measurements with visible and infrared observations. This technique is applicable both day and night times under all-weather conditions for monitoring vegetation biomass and ecosystem exchange processes, particularly under cloudy conditions where classic optical indexes are unavailable. The EDVI values represent physical properties of crown vegetation such as vegetation water content of crown canopies. The EDVI values are statistically sensitive to evapotranspiration fractions (EF) with a correlation coefficient (R) greater than 0.79 under all-sky conditions. For clear skies, EDVI estimates exhibit a stronger relationship with EF than normalized difference vegetation index (NDVI). Further applications to Amazon Basin show that microwave based EDVI can provide the vegetation information over 99% of the land surface while only small fraction (20%) of land surface information can be monitored by the classic vegetation indexes.