Irrigation efficiency is defined as the ratio of irrigation water consumed by the crops to the water diverted (W_g) from a river or reservoir or wells. This terminology serves for better irrigation systems designation and irrigation management practices improvement. But it is hard or high cost with labor intensity to estimate irrigation efficiency from field measurement. This paper proposes an estimating method of irrigation efficiency at the scale of irrigation district based on ET from remote sensing with ETWatch, a system to monitoring ET with multi-source remote sensing data, and data on water diversion: (1) ET monitoring: ET is actual water consumption of crops in the field. Irrigation water can be divided into evapotranspiration of crops and leakage to groundwater or return flow to surface water. To acquire high accuracy ET dataset is the key step for calculating the value of irrigation efficiency. Once ET data is available, it is not necessary to acquire leakage data or return flow. ETWatch with annual deviations 3.8% at sub-basin scale has been used to monitor ET; (2) Irrigation area estimation: actual irrigated areas vary with rainfall, considering rainfall is the only available water for non-irrigated field so that ET from those fields should less than irrigation area given same crop type and phenology. Irrigation area can be retrieved by using ET (ET large than rainfall) with field verification; (3) Crop classification: due to the difference of phenology, crops would show different NDVI profile, so that crop types could be distinguish through constructing NDVI time series dataset, which further divided into non-irrigated and irrigated crops. (4) Ration of ET contributed by irrigation water: Farm process model is applied to estimate transpiration ration (T_irri) from irrigation water, taking account of precipitation, soil types, irrigation water and crop phenology; (5) irrigation efficiency estimation: the summary of ET_irri of different crops is water effective consumed of irrigation water, so irrigation efficiency of irrigation district could be calculate as following formulation IE=T_irri/W_g.