Abstract: The objective of this paper is to explore the assessment indicators and issues of scale effect of irrigation water saving, which caused by the return flows and their reuse in water resource system. The authors analyzed the components of water balance and their roles at different scales, and pointed out that the situation would be becoming more complicated and scale effect could be more and more significant with the increase of scale. The classical concepts of irrigation efficiency, which ignored the existence of scale effect, had limits and constraints in their usefulness in the field of water resource management and assessment, so some new definitions and indicators introduced recently were reviewed. The changing law of assessment indicators with scale and water saving strategies of irrigation district scale were discussed through the analysis and comparison of two cases.