As the basic structural unit of soil, aggregates have different microbial community compositions in different particle sizes, which will inevitably affect the organic carbon content in aggregates. However, further research is needed on the relationship between organic carbon and microorganisms at the soil aggregate level. Based on this, this study takes the grassland in the loess hilly area that has been abandoned for 10-50 years as the research object, and explores the relationship between changes in soil aggregate organic carbon content, carbon pool stability index, and microbial community. The results showed that: (1) With the increase of years of returning farmland, the organic carbon content of soil aggregates showed an upward trend, reaching its maximum at 50 years of returning farmland. Among all particle sizes, the 0.20-2mm particle size has the highest content, while the<0.053mm particle size has the lowest content; (2) The Carbon Pool Management Index (CPMI) shows an upward trend with the increase of years of returning farmland, reaching its maximum at 50 years of returning farmland. Among different particle sizes, CPMI shows a decreasing trend as the particle size decreases. (3) There are differences in the composition of microbial communities in soil aggregates at different particle sizes during the process of grassland conversion. Bacteria are mainly Proteobacteria and Actinobacteriota, while fungi are mainly Ascomycota and Mortierellomycota; (4) The effect of microbial community composition on organic carbon content varies among different particle sizes. Bacteria play a major role in>2mm particle size aggregates, while bacteria and fungi work together in 0.20-2mm and 0.053-0.20mm particle sizes. Fungi are dominant in<0.053mm particle size aggregates; (5) The key microbial groups that affect organic carbon content vary in particle size, with Firmicutes in the>2mm and 0.053-0.20mm particle sizes, Chloroflexi in the 0.20-2mm particle size size, and unclassified_k__Bacteria in the<0.053mm particle size. This study can provide a scientific basis for the assessment of carbon sequestration effects and carbon pool management of converted grasslands in loess hilly areas.