Abstract: A comprehensive review of scale effect (SE)on the irrigation water use efficiency (IWUE)is presented in this study,including the definition and its causes as well as the upscaling techniques.SE is the composite outcome of two main causes that refer to reusing irrigation return flow and the spatial variability of local properties such as soil, crop,climate,irrigation structures,and amount of irrigation water and others.The existing indices of irrigation water use efficiency should be extended to including the connotation of water supply efficiency.Reusing irrigation return flow should be considered together with the economic constraint.The existing studies on scale effects show that the regularity with regard to indices of irrigation water use efficiency is not conclusive due to the complexity of nature conditions.Regarding to upscaling problem,the two main causes have been taken into account separately and need to be synthesized in the future studies.A reliable and practical upscaling formula is still unavailable for use to improve the practice of water management.Many efforts should be devoted to the theory of scaling as well as the scale effect of accompanying courses with the water cycle,based on the improvement of irrigation water use efficiency assessment indices.