ABSTRACT Due to large-scale wetland reclamation, the typical wetland had been conversed to different wetland use types (upland field, paddy field, and artificial forest) in the Sanjiang Plain. However, there are scarce data regarding soil aggregates and active organic carbons during land-use transition. Here, soil aggregates and the changes in content and storage of active organic carbon [total organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), and readily oxidized carbon (ROC)] were studied under three land-uses reclaiming wetlands as an upland field, paddy field, and artificial forest in Heilongjiang Province, China. The results showed that soil aggregate structure changed significantly under the three land-uses, of which the largest decrease of mean weight diameter (MWD) occurred in the upland field. Under the three land-use types, the content and storage of TOC and each active organic carbon in soil aggregates with different size fractions significantly decreased compared with that in the wetland. In addition, the proportion of the 1–2 mm soil aggregate was significantly lower than that of other particle sizes, which resulted in the lowest storage of TOC and active organic carbon at 1-2 mm and was found in the wetland, and different land-use types; small aggregates (<0.25 mm) with a small proportion were ignored in calculating organic carbon content and storage. The TOC and active organic carbon content in various soil aggregates varied significantly in different land-use types; the paddy field was most similar to the wetland because of seasonal flooding factors. In soil aggregates with various size fractions, the paddy field presented small changes in the TOC, DOC, and MBC content, and the ROC content was significantly lower than that in the upland field and the artificial forest land-use types. The TOC, DOC, and MBC content in the upland field and the artificial forest were significantly reduced compared with that in the paddy field due to the decrease of water content in the soil. In the artificial forest, which had less human disturbance, organic carbon content was less affected than in long-term cultivated upland fields due to its abundant plant root systems and large input of organic matter. The upland field could seriously affect the structure of soil aggregates and organic carbon in the wetlands and had the most negative impact on the wetland ecosystem.