Abstract: The spatial variation of soil organic carbon (SOC) of croplands and its influencing factors at county scale were studied in Zhuanglang County in southern Gansu Province by using a geo-statistics method combined with Geographic Information System (GIS). The results show that the SOC are low with a mean value of 8.28g/kg, and the coefficient of variation of SOC is 18.8%, which expresses a medium spatial variation. The result of semi-variograms indicates that the rate of nugget to sill is 9% showing a strongly spatial self-relativity of SOC in this area. The range of spatially dependent for SOC is 2250m. The Kriging spatial interpolation at county scale show that the SOC contents in the north part are higher than those in the south part, the SOC contents in the west part are higher than those in the east part, and some high values are in patch forms in the west part. At county scale the main influencing factors for the spatial the SOC contents of farmland SOC are altitude and soil type, and altitude could explain 82.27% of variability, soil type 13.18%, aspect and soil erosion degree 4.54%.