Based on a long-term fertilization experiment of paddy field setup in 1990 and located in Taoyuan County of Hunan Province, the effects of fertilization and straw returning on soil organic carbon (SOC) distribution over soil depth, density and variation, stable carbon isotope natural abundance (δ13C), and mineralization characteristics of organic carbon were studied. Results showed that long-term amendments of fertilizer and crop straw returning significantly increased SOC in the top soil, straw returning were better than chemical fertilizer in organic carbon accumulation; SOC density of chemical fertilizer and crop straw treatments in 0 - 25 cm and 0 - 50 cm were significantly increased, but variation range of SOC density was also increased with long-term amendments; Significant differences of the δ13C with CK were found in 0 - 10 cm soil layer, but no significant differences among chemical fertilizer treatments and crop straw treatments; Organic carbon mineralization of different treatments reached the peak in the initial stage, and gradually leveled off and stable in 1 - 3 weeks followed; The average mineralization rates varied in the range of 47.75 - 31.16 CO2 ml/(kg·d) and stable mineralization rates in the range of 40 - 60 CO2 ml/(kg·d), cumulative mineralization amounts of different treatments in 12 weeks were in the decreasing order of CK+C＞NP+C＞N+C＞NPK+C＞NP＞NPK＞N＞NK＞CK; Amendments of fertilizer and crop straws increased SOC cumulative mineralization, but the effects of crop straws returning were more significant than chemical fertilizer; Therefore, as a way of SOC accumulation long-term amendments of chemical fertilizer and crop straws improved supplying capability of soil nutrient, and benefited for buffering and stabilizing organic carbon pool in paddy soil ecosystem.