We analyzed the content and density of soil organic carbon along a chronosequence of Pinus tabulaeformis plantations (9, 23, 33 and 47 years old) in the Loess Hilly Region, and explored the relationships between soil organic carbon content and main factors affecting it, i. e. , total nitrogen, soil particle density, root biomass and litter biomass. The results showed that the average contents of soil organic carbon were 4.9, 5.9, 9.2 and 6.5 g/ kg,and the densities of soil organic carbon were 63.0, 66.8, 100.7 and 72.5 mg/ hm², for 9-, 23-, 33-and 47-year-old plantations, respectively. Both the content and density of soil organic carbon increased first (9 - 33 years) and then decreased (33 - 47 years) over stand age. In addition, the content and density of soil organic carbon decreased gradually as soil depth increased. The density of organic carbon in surface soil layer (0 - 30 cm) accounted for 48.5% - 57.9% of that of the entire observed soil layer. 2) The soil organic carbon content had a significantly positive correlation with total soil nitrogen, root biomass and litter biomass. However, there was a significantly negative relation between soil organic carbon and soil particle density. The correlation coefficients between soil organic carbon content and root biomass and litter biomass decreased gradually with soil depth increasing. The result of stepwise regression indicated that total soil nitrogen and litter biomass were the main factors affecting soil organic carbon content.