The dynamics of soil organic carbon (SOC) stock in croplands may have significant effects on the global carbon budget. The regional and/or national SOC stock dynamics of China's cropland soils, which have been under agricultural management practices during the last three decades, have been actively debated. In this study, we estimated the magnitude of topsoil SOC stock in typical croplands of the Songnen Plain of Northeast China, using samples from 1460 sites during 2008-2011. We also examined the change of topsoil SOC stock by comparing our current measures with those of 344 profiles derived from China's National Soil Inventory during the 1980s. The total amount of SOC stock in the upper 20 cm croplands of study area was estimated to be 48.72 Tg (1 Tg = 1012 g), with an average density (carbon stock per area) of 3.76 kg C m(-2). Site-level comparisons indicate that the topsoil SOC stock has increased significantly over the past three decades, with an average increase of 0.56 kg C m-2, ranging from 0.24 to 0.88 kg C m(-2). In the case of croplands in Northeast China, we speculate that the increased soil C input due to increased crop productivity and the return of corn stubble into the soil might be the most crucial factors. Our results suggest that cropland management plays an important role for carbon sequestration to mitigate climate change in this important agricultural region.