【Objective】 Forest soil organic carbon (SOC) is an important component of the carbon storage in the forest ecosystem. Accumulation and storage of SOC in the forest soil can directly affect carbon budgeting in the terrestrial ecosystem and global carbon recycling. SOC in mountain forest soils in the transitional climatic zone is sensitive to climatic change. So the subject of variation of SOC along an altitudinal gradient is also a key topic concerning regional carbon dynamics. Hence, the survey of altitudinal gradient of SOC in a transitional climate zone is far more valuable than that in a typical climate zone to predicting variation of regional SOC in response to climate changes. The objective of this study was to investigate effects of altitude on SOC so as to explain how carbon stocks distributes in the subtropical transitional climate zone of East China. 【Method】 A total of 20 soil profiles were prepared along the altitude (179-1 410 m a.s.l.) in Jiangxi Jiulianshan National Nature Reserve, located just in the transitional climate zone from the south-subtropical zone to the mid-subtropical zone. Soil samples were collected separately from five layers (0-10 cm, 10-20 cm, 20-40 cm, 40-60 cm and 60-100 cm) of each of the soil profiles (0-100 cm). After being air-dried, the samples were analyzed for soil organic carbon content with the potassium bichromate oxidation titration method. Soil organic carbon density (SCOD) and soil organic carbon stocks (SOCS) were calculated. Along the altitude, red soil, red-yellow soil, yellow soil and meadow soil was distributed in sequence, so impacts of altitude on SOCC and SBD (soil bulk density) could be explored by analyzing the two parameters in the four types of soils. Soil layer depth and soil type were two factors affecting SOCC and SBD and their impacts were analyzed with two-way Analysis of Variance (two way- ANOVA); impact of soil type on SOCD was with ANOVA; and relationships of altitude with SOCC and SOCD were with Pearson correlation analysis and nonlinear analysis, respectively. 【Result】 Results show: (1) SBD increased dramatically with soil depth, but did not vary much between soil types; soil type and soil layer depth influenced SOCC, with the mountain meadow soil being the highest in SOCC among the four types and varying in the range of 39.72 ± 19.14 g•kg^-1; SOC accumulated mainly in the topsoil; SOCC in 0-0 cm was fourfold of that in 60-100cm; and 84.74% of the SOC in the soil were accumulated in the 0-40cm soil layer of the soil profile. (2) Topsoil was more sensitive than the other soil layers to change in altitude, and the effect of altitude on SOCC declined with soil depth; SOCC in the 0- 40 cm topsoil increased with altitude, but in the 40-100 cm subsoil layer, SOCC acted reversely. (3) ANOVA shows that soil type did not contribute much to soil carbon density, so SOCD was estimated at 10.64 ± 0.72 kg•m^-2 and SOCS at 1.426 ± 0.096 Pg in the soil of the nature reserve. Pearson correlation analysis and nonlinear analysis shows that the relationship between altitude and SOCD could be expressed as SOCD = 1×10^-5 altitude² - 0.0227 altitude+ 17.928. 【Conclusion】 The findings in the study lead to the conclusion that the effects of altitude on SOC spatial distribution is quite complicated. Soil organic carbon accumulates mostly in the topsoil layer (0- 40 cm) of the soil profiles. Topsoil SOCC increases with altitude, but the trend weakens with soil depth. However, altitude does not have any impact on SOCD.