[Objective] To investigate the effects of grassland reclamation on the variation of soil aggregates and organic carbon distribution, in order to provide theoretical basis for land use optimization and grassland protection of Siziwang Banner, Inner Mongolia. [Methods] Three soil types (chestnut soil, gray cinnamon soil and meadow soil) were selected from the grassland and cultivated land in the farming-pastoral ecotone of Siziwang Banner, Inner Mongolia. The composition of soil aggregates, soil organic carbon, and soil organic carbon of aggregates with different particle sizes and their contribution rate in the 0-10 cm soil layer were analyzed. [Results] The content of aggregates with particle sizes >3 mm and 0.25～3 mm were lower, but the content of aggregates with particle size ≤0.25 mm was higher in cultivated land than in grassland with chestnut soil and gray cinnamon soil. Moreover, and soil organic carbon of each soil aggregates with different particle size were lower in cultivated land, compared with the other two soil types. The main body of organ carbon storage transformed from soil aggregate with particle size >3 mm to 0.25～3 mm soil aggregate. The contribution rate of non-aggregates to soil organic carbon was increased in the cultivated land soil. The structure of meadow soil was poor with low organic carbon content, which was mainly caused by the salinization, but it was improved by the long-term cultivation and application of organic fertilizer after reclamation for arable land. [Conclusion] After reclamation of the grassland with chestnut soil and gray cinnamon soil into cultivated land, soil structure was degraded and the stability of soil organic carbon was decreased, however, proper reclamation in meadow soil was feasible under suitable conditions.