Abstract: The objectives of this study are is to present the applications and assumptions and problems in the application of isotope tracer in watershed hydrological modeling and promote new advances and new research directions.An evaluation and review of the application of isotope tracer in watershed hydrological modeling are presented in the paper.It is motivated by new and emerging interests in isotope tracer application in hydrological modeling and the need to classify different directions.The review is focused on three issues including:(1) isotopic hydrograph separation,(2) mean residence time and mixing,and(3) coupled with hydrological model.Additionally,the review provides a critical analysis of unresoloved issues when applied in watershed hydrological modeling.These issues include:(1) temporal and spatial variation of isotopic concentration of precipitation,(2) sampling of different runoff,(3) spatial variation of isotopic concentration of different runoff,(4) the distribution function of mean residence time,and(5) coupled hydrological and isotopic model.