Increasing concentrations of atmospheric CO₂ and other greenhouse gases are widely recognized to be contributing to the rising average temperature of Earth's atmosphere since the late 19th century. Urban forests have great potential to affect global warming by removing greenhouse gases and sequestrate carbon into its biomass and the soils. Carbon storage and its special distribution in urban forests are important indicators to accurately determine carbon sequestration capacity in urban ecosystems, to completely evaluate urban ecosystems' functions and services, and to efficiently develop sustainable urban 'carbon forests’ management. To improve understanding of the influence of urban forest structure on carbon pools in urban ecosystems, the content, storage and spatial distribution of carbon in a 13-year-old Pinus massoniana forest ecosystem were investigated in Changsha City, Hunan Province, China. Results showed that carbon concentrations varied with tree organs in the same tree species and with tree species in the same tree organ as well in the Pinus massoniana forests.The average carbon content was 511.17 g/kg for different organs of Pinus massoniana forest and decreased in the order leaf > trunk > root > bark > branch. The amount of carbon stored in shrub, herb and litter-fall layers was 531.66, 465.53 and 393.92 g/kg, respectively. Carbon content ranged from 9.40 to 24.73 g/kg in the soils and gradually decreased with the increase of soil depth. The ecosystem stored 159.93 t C/hm², of which 20.1% in above-ground part (living and dead vegetation) and 79.9% in below-ground part (roots and soil organic matter in 0-60 cm depth). Carbon stocks in the different components of the studied forest ecosystems were in an order as: soil ＞vegetation＞ litterfall.The vegetation component stored 34.50 t C/hm² and represented 21.57% of the total carbon storage in the forest ecosystem. Carbon storage was about 32.4 t/hm² in the overstory layer, which accounted for 20.27% of the total amount of carbon in the ecosystem, and approximately represented 94% of carbon in vegetation component. Of the overstory layer, 65.5% was in stemwood, 19.2% was in roots, 13.2% was in branch and leaves, and 2.1% was in stembark. Carbon storage in litter fall layer was 3.81 t/hm²,which accounted for 2.38% of the total carbon storage in the ecosystem. The soil was the largest of carbon storage component (121.62 t/hm² in 0-60 cm depth) and represented about 76% of the total ecosystem carbon storage. The annual net primary productivity was estimated to be 4.88 t·hm^-2·a^-1 in the Pinus massoniana forests and annual carbon storage was 2.50 t·hm^-2·a^-1, which was equivalent to approximately 9.2 t·hm^-2·a^-1 of carbon dioxide. In other words, the studied 13-year-old Pinus massoniana forest ecosystem has the capacity to remove about 9 t CO₂ from the atmosphere in Changsha City. Our study demonstrated the important role played by forests in mitigating global climate change, and the research results would provide scientific reference for accurately evaluating carbon balance of urban forest ecosystems.