This study was carried out in the Jigong Mountain, a temperate-subtropical ecotone, where there is extensive mixed pine-oak forest. Three study sites were established at the altitudes of 200, 400, and 600 m, respectively. The soil carbon density and total carbon storage in the mixed pine-oak forest were determined. The carbon storage in the vegetation layer under the canopy and the litter were also measured. The biomass and carbon storage of the tree layer was estimated using biomass equations. The carbon storage of the mixed pine-oak forest was compared with that of the deciduous oak forest. The results suggested that the total carbon storage of the mixed pine-oak forest ecosystem is 179.74 t·hm^-2 and the carbon storage in each individual layers of the mixed pine-oak forest was different with a decreasing order of tree layer (97.57 t·hm^-2) > soil layer (70.56 t·hm^-2) > litter layer (10.57 t·hm^-2) > shrub layer (0.83 t·hm^-2) > herb layer (0.21 t·hm^-2). However, the stored carbon contents at the layers (except for soil layer) with different altitudes were not significantly different. Soil carbon storage significantly increased with an increase of the altitude and decreased with the increasing depth (P < 0.05). We also found that carbon storage of the mixed pine-oak forest was positively related to forest canopy and it was higher than that of the deciduous oak forest. Those results help us understand the spatial distribution characteristics of the mixed pine-oak forest ecosystem and can facilitate the management?of the forest as a?carbon?sink.