Greening tree species in urban is vital part of urban ecosystem. To study effects of seasonal variation and other variables on water use efficiency the tree species, 24 trees species (6 evergreen conifer and 18 deciduous broadleaf trees) were used to examined seasonal change in the foliar δ13C and relationships of the δ13C with soil temperature, humidity and the other weather variables in Beijing Botanical Garden in 2006， The results showed that the foliar δ13C of evergreen species had insignificant difference,and the seasonal change wad from -27.0‰ to -24.5‰, however, the foliar δ13C these deciduous species and the seasonal change were insignificantly different（p=5.05×10-6