This study analyzed the hydrogen and oxygen isotope characteristics of waters from rain, spring, ground and soil, and xylem water of Quercus variabilis and Grewia biloba in the southern lithoid hilly area of the Taihang Mountains, China. Water use patterns of Q.variabilis and G. biloba were determined with the IsoSource model and leaf transpiration strategyof the two species was analyzed. The results indicated significant difference between δ¹⁸O and δ D values of the xylem water of the two species in the same ecosystem. Q.variabilis and G. biloba mainly used shallow water from 0-20 cm soil layer during the wet season, while both species uptook deep soil water during the dry season. In the dry season, Q.variabilis mainly used water from 40-60 cm soil layer, while G. biloba mainly used water from 20-40 cm soil layer. Other than that, Q.variabilis used more spring water than G. biloba, accounting for 19.6% of its total water usage during the dry season. So, Q.variabilis and G. biloba could avoid water competition due to the different water sources for the two species in the dry season. Q. variabilis, which occupied the upper canopy layer in the ecosystem, showed midday depression of transpiration that was caused by partially closed stomata at noon during the dry season. Meanwhile, the transpiration rate and stomatal conductance of shaded G. biloba were 46.94% and 30.58% lower than that of Q. variabilis, respectively. With different strategies, Q.variabilis by using deep soil water and partially closed stomata, and G. biloba by using shallow soil water and decreasing transpiration, the two species used the limited water sources reasonably during the dry season, as the result the ecosystem composed of these two species showed strong ecological adaptability to the fragile environment in the southern lithoid hilly area of the Taihang Mountains.