The Loess Plateau is an area of low crop productivity and one of the most susceptible areas for soil erosion in China. This study was designed to investigate changes in soil organic carbon (SOC) of croplands in four humid to semiarid counties (Huixian, Xifeng, Linxia, and Yuzhong) of Gansu province that have experienced accelerated grain production since the early 1980s. In 1979 to 1988 and 2006 to 2007, respectively, 2036 and 15,764 topsoil (20-cm) samples were collected and analyzed for SOC concentration and density from croplands covering 12 Chinese soil groups. By comparing the SOC data from the mid-2000s and the 1980s, we found that across the four counties, topsoil (20-cm) SOC concentration and density in croplands changed significantly (P <0.01) at a rate of 0.04 ± 0.14 g kg-1 yr-1 or 0.01 ± 0.03 kg m-2 yr-1 from 7.9 ± 3.5 g kg-1 and 1.98 ± 0.9 kg m-2 in the 1980s to 8.7 ± 2.4 g kg-1 and 2.22 ± 0.5 kg m-2 in the mid-2000s, respectively. This overall increase in topsoil (20-cm) SOC concentration by 10% or density by 12% in the mid-2000s was associated with an increase of 83% in the stubble and root inputs to the soil across the four counties. There were large differences in the change size in the cropland topsoil (20-cm) SOC concentration and density among counties. In Huixian and Xifeng, both the topsoil SOC concentration and density in the mid-2000s were significantly higher than in the 1980s (P <0.01), whereas there were no significant changes in topsoil SOC concentration and density in either Linxia or Yuzhong. Soils with lower initial SOC tended to increase more in SOC concentration than soils with higher initial SOC. © Soil Science Society of America. All rights reserved.