Leaf carbon isotope composition (δ¹³C) of both vascular and non-vascular plants were investigated in order to assess their variability along an altitude gradient (414, 620, 850, 1086, 1286 and 1462 m) from a subtropical monsoon forest located at Mt. Tianmu Reserve, eastern China. Leaf δ¹³C values of all plant species ranged from -34.4 to -26.6‰, with an average of -29.8‰. There is no significant difference in leaf δ¹³C between vascular plants and mosses, however, trees had significantly higher δ¹³C values than herbs. For pooled data, leaf δ¹³C was positively correlated with altitude. Leaf δ¹³C was significantly and negatively correlated with annual mean temperature and atmospheric pressure, while it was significantly and positively correlated with soil water content. Furthermore, there was no relationship between leaf δ¹³C and soil nitrogen content or soil phosphorus content. The altitudinal trend in leaf δ¹³C is the consequence of the interaction between temperature, atmospheric pressure and soil water content.