A process-based model (Agricultural Production systems SIMulator, APSIM), which was developed in Australia, was used to study the spatiotemporal changes in agricultural soil organic carbon (SOC) across the North China Plain (NCP) from 1980 to 2010. The modeling results suggested that the calibrated APSIM model generally performed well in terms of simulating changes in yields of wheat and maize and SOC under different treatments at three selected long-term experimental sites: Xuzhou, Zhengzhou, and Changping. The regional simulations showed that most croplands of NCP experienced an increase in SOC, except several areas of the northern part of Hebei Province, and central and eastern parts of Shandong Province. The average rate of change of SOC was 0.35 Mg(C) ha^-1 a^-1, and the total increase in the SOC pool was 257.43 Tg (with a total cropland area of~24.52 Mha in NCP). Spatially, the cropland SOC pool increased by 102.05, 59.82, and 95.56 Tg in the Beijing-Tianjin-Hebei area, Shandong Province, and Henan Province, respectively. The increase in cropland SOC was mainly attributed to the increasing amount of carbon input during the past several decades.