Soil organic carbon (SOC) is one of the centre issues related to not only soil fertility but also environmental safety. Assessing SOC dynamics in croplands has been a challenge in China for long due to the lack of appropriate methodologies and data sources. As an alternative approach for studying SOC dynamics, process-based models are adopted to meet the needs. In this paper, a process-based model, DeNitrification-DeComposition (DNDC), was applied to quantify the SOC storage and the spatial distribution in croplands of China in 2003, with the support of a newly compiled county-level soil/ climate/land use database. The simulated results showed that the total SOC storage in the top layer (0-30 cm) of the 1.18 × 108 ha croplands of China is 4.7-5.2 Pg C in 2003 with an average value of 4.95 Pg C. The SOC storage in the northeastern provinces (1.3 Pg C) accounts for about 1/4 of the whole national totals due to their dominantly fertile soils with high organic matter content. SOC density ranges from 3.9 to 4.4 kg C m-2, with an average of 4.2 kg C m-2, a level is much lower than the world average level. The model results also indicated that high rates of SOC losses occurred in the croplands with the most common cropping patterns in China as like single soybean > maize > paddy > cotton > winter wheat and corn rotation. The results reported in this paper showed that there was still a great potential for improving SOC status in most croplands of China by adopting proper farming practices and land-use pattern. Therefore, long-term policy to protect SOC is urgently needed. Assessing SOC dynamics in croplands has been a challenge in China for long due to the lack of appropriate methodologies and data sources. As an alternative approach for studying SOC dynamics, process-based models are adopted to meet the needs. In this paper, a process-based model, DeNitrification-DeComposition (DNDC), was applied to quantify the SOC storage and the spatial distribution in croplands of China in 2003, with the support of a newly compiled county-level soil / climate / land use database. The simulated results showed that the total SOC storage in the top layer (0-30 cm) of the 1.18 × 108 ha croplands of China is 4.7 -5.2 Pg C in 2003 with an average value of 4.95 Pg C. The SOC storage in the northeastern provinces (1.3 Pg C) accounts for about 1/4 of the whole national totals due to their dominantly fertile soils with high organic matter content. SOC density The model results also indicates that high rates of SOC in the croplands with the range from 3.9 to 4.4 kg C m-2, with an average of 4.2 kg C m-2, a level is much lower than the world average level. most common cropping patterns in China as like single soybean> maize> paddy> cotton> winter wheat and corn rotation. The results reported in this paper showed that there was still a great potential for improving status of SOC in in croplands of China by suitably proper farming practices and land-use pattern. Thus, long-term policy to protect SOC is urgently needed.