The conversion of a natural ecosystem to cropland strongly influences soil properties and unsuitable practices lead to degradation of soil end environment quality. The objective of this study was to assess the impact of different land uses on physical properties and organic carbon pools of an Oxisol in a tropical environment of Brazil (21°14′S, 48°17′W). Sites, which were converted from natural ecosystem to crop land or pasture, were established. Within the same Oxisol, areas with different land uses were compared, including a natural ecosystem (Tropical Forest), pasture for 35 years, conventional tillage for 35 years, and no tillage for 20 years, after 15 years of conventional tillage. The soil was sampled at 0.0-0.10 m depth and analyzed for total organic carbon (TOC), humic acids (C-HA), fulvic acids (C-FA), humin (C-HUM) contents, particulate organic C (POC), soil mineral associated organic C (MAC), mean-weight diameter (MWD) of water-stable aggregates, and aggregate stability index (ASI). Soil under natural ecosystem and pasture contained the largest amount of TOC, C-HA, C-FA, C-HUM, POC, MAC, and the highest MWD and ASI values, and the conventional tillage the smallest. The decrease in TOC from natural ecosystem to conventional tillage was 65% and from no tillage 55%. Then, the increase of TOC from conventional to no tillage was 15% in 15 years. The TOC under pasture was similar to those under natural ecosystem; however, C-HA under pasture was higher. The MWD decrease by 60% after conversion of the soil from forest to crop land under conventional tillage. Soil under no tillage for 20 years, showed similar values of MWD and ASI to those of the natural ecosystem. The results showed that no tillage has the potential to recuperate soil structure even though presenting lower organic soil content. The conventional tillage proved to be the least sustainable land use.