Abstract: Soil organic matter is an important component in soil and plays an important role in soil fertility, environmental conservation, and sustainable agricultural development. Aggregate is an essential part of soil structure and the key material basis for good soil structure, which comprehensively reflects the status of soil fertility. The size and content of soil aggregate have become indicators for evaluating soil quality level and anti-erosion ability. Recently, the research on the relationship between soil organic matter and aggregate as well as its influencing factors has attracted more attention. Based on four typical fertilization treatments (MNPK-mixed application of organic and chemical fertilizers, NPK-application of chemical fertilizers only, M-application of organic fertilizer only, CK-non-fertilization) under long-term fertilization experiment in Mollisols, we analyzed the size composition of soil aggregates and their soil organic carbon (SOC) contribution rates to total SOC. The results showed that Mollisols, under different long-term fertilization systems, the sizes of 2 mm~0.25 mm and 0.25 mm~0.053 mm aggregates, at 0~20 cm soil depth were the main components, accounting for 46.5% and 39.6% in total soil aggregate, respectively. The SOC content varied among different size soil aggregates. The highest was found in 0.25 mm~0.053 mm aggregate but the lowest in <0.053 mm aggregate. Compared with CK, the combined application of organic and inorganic fertilizer (MNPK) increased SOC content in different size aggregates to some extent. The contribution rate of 2 mm~0.25 mm aggregate in total SOC was 53.0%, 45.7% and 51.1% in MNPK, M, and CK, respectively, while the contribution rate of 0.25 mm~0.053 mm aggregate was 47.9%, with the highest contribution rate in total SOC in chemical fertilizer treatment (NPK). The above mentioned was consistent with the aggregate size proportion in long-term fertilization systems.