Abstract: In order to examine the effects of soil organic carbon sequestration under different plantation structures in the tablelands of Loess Plateau, in this paper, Biota orientalis, Robinia pseudoacacia, Hippophae reamnoides and Pinus tabuliformis pure and mixed planting models were selected. The soil organic carbon (SOC), easily oxidizable organic carbon (EOC), light fraction organic carbon (LFOC), particulate organic carbon (POC) and dissolved organic carbon (DOC) were analyzed. The results show that the accumulation of carbon in the mixed forestlands is higher than that in the pure forestlands. Compared with the pure forestlands, the mixed forests of Biota orientalisRobinia pseudoacacia could improve soil labile organic carbon (LOC) contents in the 0-5 cm soil layers effectively, for example, SOC are increased by 123% and 160%, EOC are increased by 161% and 304%, LFOC are increased by 1399% and 482%, POC are increased by 454% and 436%, and DOC are increased by 138% and 170%. While the improvements in the mixed forests of Robinia pseudoacacia-Hippophae reamnoides, Pinus tabuliformis-Hippophae reamnoides-Robinia pseudoacacia are small. Among the soil LOC contents, compared with the pure forests, the soil EOC contents are improved most significantly in the mixed forests, LFOC contents are the next, while the soil POC contents are not significantly improved. The soil LFOC is more sensitive than others. In the 0-100 cm soil profile, the sensitivity index of LFOC is 1.00~10.58, 3.83~10.19 and 1.78~5.10 times of those of the SOC, DOC and EOC respectively. The correlation coefficient and regression analysis of the organic carbon fractions show that there is a very significant linear positive correlation between SOC and LOC, and among soil organic carbon fractions, POC accounts for the highest percentage, LFOC for next, while DOC just a little. In addition, the correlation coefficient of between soil LFOC and POC is higher than those between soil LFOC and the others, and the correlation coefficient of between soil DOC and EOC is higher than those between soil DOC and the others.