It is difficult to estimate soil thickness eroded from annual erosion rates in cultivated Black soils in northeast China because of the uncertainty of the time when the soil was first cultivated for agricultural crops. Assuming soil organic carbon (SOC) profile curves for cultivated sites are the same as virgin sites before cultivation, it may be possible to estimate the total thickness of surface soils lost by erosion by vertical movement of plotted SOC profiles until those for the virgin and cultivated soils are superimposed. We collected pairs of soil samples (0–1 m) with one sample in each pair from a virgin site and the other from a nearby cultivated site in Heilongjiang province, northeast China. In undulating areas where soil erosion was moderate, the shapes of SOC distribution curves below 40 cm depth were nearly identical for both cultivated and virgin soils, but were offset vertically. This offset was attributed to the loss of surface soil by erosion in the cultivated land. By moving the distribution curve of SOC in cultivated soil downwards by 12.7 cm, we found nearly coincident curves below 45 cm for the virgin and cultivated soils. This thickness (12.7 cm) was believed to be the depth of soils that had been eroded since the onset of cultivation in Black soils in northeast China. We concluded that the amount of surface soil lost by erosion could be estimated from comparison of the vertical distribution of SOC in cultivated and virgin soils.