In this study, a coniferous tree species (Pinus tabuliformis Carr.) was investigated at a moderate-altitude mountainous terrain on the southern slope of the middle Qinling Mountains (QLM) to detect the trends in carbon isotope ratio (delta C-13), leaf nitrogen content (LNC) and stomatal density (SD) with altitude variation in north subtropical humid mountain climate zone of China. The results showed that LNC and SD both significantly increased linearly along the altitudinal gradient ranging from moo to 2200 m, whereas leaf delta C-13 exhibited a significantly negative correlation with the altitude. Such a correlation pattern differs obviously from that obtained in offshore low-altitude humid environment or inland high-altitude semi-arid environment, suggesting that the pattern of increasing delta C-13 with the altitude cannot be generalized. The negative correlation between delta C-13 and altitude might be attributed mainly to the strengthening of carbon isotope fractionation in plants caused by increasing precipitation with altitude. Furthermore, there was a remarkable negative correlation between leaf delta C-13 and LNC. One possible reason was that increasing precipitation that operates to increase isotopic discrimination with altitude overtook the LNC in determining the sign of leaf delta C-13. The significant negative correlation between leaf delta C-13 and SD over altitudes was also found in the present study, indicating that increases in SD with altitude would reduce, rather than enhance plant delta C-13 values.