Plant carbon isotope discrimination (δ¹³C) has been one of core issues in fields of plant ecology and global carbon cycle since ¹³C of plant tissue was found to be more than that of atmosphere. Many researches have found that plant (especially C₃ plants) δ¹³C increased with altitudes. Discrimination of photosynthesis to ¹³C, which is a key process for causing the difference of ¹³C content between plants and atmosphere, is influenced by environment, but it achieves through biological processes finally. So analysis of relationships between plant δ¹³C and eco-physiological parameters (photosynthesis, diffusion, nutrient content, morphology etc.) is more meaningful and helpful to interpret the relationship between plant δ¹³C and altitudinal gradients than those between plant δ¹³C and environmental factors. Foliar traits on carbon isotope ratio, photosynthesis, diffusional conductance to CO₂, nutrient content and morphology of Quercus spinosa in three sites with different altitudes from Wolong reserve were measured in order to understand and interpret how foliar δ¹³C of Quercus spinosa responses to altitudinal gradients in this area. The relationships among foliar δ¹³C and altitudinal gradients and eco-physiological parameters were analyzed by Pearson correlation analysis firstly. Some of them that correlated significantly with each other were analyzed again by Standardised Major Axis (SMA). Foliar δ¹³C of Quercus spinosa increased with altitude (R²=0.56, P<0.001), and the altitudinal difference of δ¹³C amount to 2.0‰ per 1000m. Theoretically, when mesophyll conductance (g_m) is considered as a limited factor for CO₂ diffusion, there should be a more significant correlation between δ¹³C and the ratio of chloroplast partial pressure of CO₂ to ambient CO₂ partial pressure (P_c/P_a) than that between δ¹³C and the ratio of intercellular to ambient partial pressure of CO₂ (P_i/P_a). So P_c/P_a should be better used for indicating δ¹³C instead of P_i/P_a. We found that lower P_c/P_a at higher altitude caused by decreasing diffusional conductance (stomatal conductance (g_s) and g_m) was main reason why foliar δ¹³C of Quercus spinosa increased with altitudes and g_s (R²=0.71, P<0.001) was a more important factor to foliar δ¹³C than g_m (R²=0.65, P=0.003). Meanwhile, leaf mass per area (LMA) that increased with altitudes also had a positive effect on this trend (R²=0.35, P=0.017). Nitrogen content per leaf area (N_area) increased with altitudes, but more nitrogen was allocated to non-photosynthetic system that caused the drop of carboxylation efficiency and photosynthetic rate at high altitudes, and then played a negative role on the changes of foliar δ¹³C with altitudes in a certain extent. Additionally, photosynthetic nitrogen use efficiency (PNUE) was testified to be a better indicator for the relationship between leaf nitrogen content and foliar CO₂ fixing, also for foliar δ¹³C than N_area and nitrogen content per leaf mass (N_mass).