Abstract: Stock density is the important factor influencing the carbon storage of forest, but currently we know little about the effects of stock density on carbon storage of forest. In order to investigate the effects of stock density on carbon storage in Fraxinus mandshurica plantations, we measured carbon storage and annual carbon accumulation in 13-year-old Fraxinus mandshurica plantations with different densities(2 200, 2 500, 4 400, 10 000 tree/ha)in Maoershan area of Heilongjiang Province, northeastern China. We adopted the method of sample-plot survey to set three plots respectively in each treatment, and estimated the carbon storage of stand and the annual net carbon fixation of tree layer. The results showed that the carbon storage in tree, litter, soil layers and ecosystem increased with stock density increasing, while the carbon storage in understory vegetation layer decreased with stand density increasing, in which there was a significant difference(P < 0.05)in tree, understory vegetation layer, soil layer and ecosystem with different densities on carbon storage, but there was no significant difference(P>0.05)in litter layer. The spatial allocation of carbon storage in plantations with four densities was soil layer > tree layer > litter layer > understory vegetation layer. The carbon storage in soil and tree layer, which were main components in carbon pool of plantations, accounted for 79.6%-82.4% and 14.1%-17.0% of the ecosystem, respectively. In addition, the annual net carbon fixation in tree layer increased with stock density increasing, which was obviously lower(P < 0.05)in plantations with 2 200 tree/ha than others. Above results suggest that improving planting density plays a significant role in increasing carbon storage of young stands.