This paper mainly studied the distributions of total organic carbon (TOC), stable organic carbon (StOC), mineral- protected organic carbon (MOC), recalcitrant organic carbon (ROC) and their correlations with the contents of clay minerals and different forms of iron oxides (Fep, Feo and Fed) for aggregates with six particle sizes of Heilu soil which is widely distributed in the Loess Plateau, and discussed the preservation mechanisms of organic carbon. The results showed that about 80% of the organic carbon existed in fraction with particle size less than 0.5 mm; the StOC was the main components of organic carbon, and it increased with the decrease of particle size; the TOC content positively correlated with the Fed content with correlation coefficients r of 0.72 (P<0.05) in 0.5–0.25 mm and 0.25–0.05 mm aggregates, and the ROC content positively correlated with the Fed content too, with r of 0.68 (P<0.05) in 0.25–0.05 mm aggregates. The contents of StOC and MOC in 0.25–0.05 mm aggregates positively correlated with the content of illite, with r of 0.71(P<0.05) and 0.68(P<0.05), respectively. Fed might maintain soil organic carbon by complexation, while illite preserves SOC by adsorption.