This paper deals with the natural abundance of ¹⁵N and ¹³C in the soil and needles of the pine forest ( Pinus roxburghii ) along an altitudinal gradient. The study area lies in the elevation range of 1200 m to 2200 m in Kathmandu valley of Nepal. The higher altitude soil was found to be much more depleted of ¹³C than the lower altitude soil. The decreasing trend of C-isotope (as well as the trend of N-isotope) might be attributed to the lower mineralization rate and net nitrification rate at the higher altitude. The observed differences in isotopic N- and C-contents of needles and soils across altitudinal gradients in this study are a result of the combination of environmentally induced variations in physiological and morphological differences. For example, altitudinal variation in needle δ¹⁵N and δ¹³C at the research site indicates that the needle isotopic composition is related to nitrogen availability at a site.