The contents and dynamics of nutrients in leaves and litter are important for plant-soil nutrient cycling in forest ecosystems. Forests are one of the most important terrestrial ecosystems in southeastern Tibet, where the litter layer develops in an alpine climate. In this study we investigated the contents and stoichiometry of the most important nutrients (carbon, nitrogen, phosphorus, and potassium) in relation to the leaf age and stage of forest litter decomposition on Sejila Mountain (elevation approximately 3400-4150 m) in southeastern Tibet. We measured the organic carbon (OC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) contents in 1-a-old and 2-a-old leaves, and at different stages of litter decomposition (non-decomposed[ND], semi-decomposed[SD], and completely decomposed[CD]), in typical forest types (Rhododendron aganniphum, Abies forrestii, and Quercus aquifolioides). The OC contents of the leaf and litter showed the following rank order:2-a-old leaf > 1-a-old leaf > ND > SD > CD (65.71%, 71.29%, 60.58%, 41.15%, and 29.86%, respectively). The litter OC content decreased with an increasing litter decomposition. The leaf OC contents were higher than the average for China (45.5%). The leaf OC contents were significantly higher in the coniferous forest than in the broad-leaved forest, and those in both forest types were higher in older leaves than in new leaves (72.89% and 58.32% for 1-a-old leaves, and 78.31% and 64.29% for 2-a-old leaves, in the coniferous and broad-leaved forests, respectively). The contents of N, P, and K in the leaf were higher in the 1-a-old leaves than in the 2-a-old leaves. The TN content of the 1-a-old leaves was higher in the broad-leaved forest (11.23 g/kg) than in the coniferous forest (10.55 g/kg), whereas the TN content of the 2-a-old leaves in the coniferous forest (9.39 g/kg) was higher than that in the broad-leaved forest (7.15 g/kg). The TN and TP contents of litter were highest in the SD forest, which indicated that net fixation occurred during initial decomposition, whereas complete decomposition resulted in a net release of nitrogen and phosphorus. The litter TK content was elevated with increased decomposition, and thus showed net fixation during the decomposition process. The litter TN contents were lower than the average for Chinese forests (12.03 g/kg), and the average values were 6.59, 8.24, and 9.55 g/kg for ND, SD, and CD forests, respectively. The litter TP contents were higher than the average for Chinese forests (0.74 g/kg), and the average values were 1.17, 2.49, and 1.87 g/kg for ND, SD, and CD forests, respectively. The litter TK contents were 2.19, 3.33, and 4.67 g/kg for ND, SD, and CD forests, respectively. The leaf C:N, C:P, and C:K ratios were higher in the 2-a-old leaves than in the 1-a-old leaves; the ratios at each leaf age were 87.81 and 60.79 for C:N, 539.25 and 375.49 for C:P, and 139.15 and 101.20 for C:K, respectively. The litter C:N, C:P, and C:K ratios were decreased with increased decomposition. The ND, SD, and CD ratios were 90.20, 43.36, and 35.68 for C:N; 520.34, 167.60, and 159.13 for C:P; and 297.73, 129.97, and 64.42 for C:K, respectively. The leaf N:P ratios were low (6.09 and 4.76), and significantly lower than the global average (16.0), which was indicative of nitrogen limitation. The litter N:P ratios for ND, CD, and SD were 5.86, 4.51, and 3.90, respectively. The results provide valuable information on plant-soil nutrient cycling in forest ecosystems in southeastern Tibet.