Abstract: The existence of row grade in the contour ridge system results in that runoff from side slopes accumulates in the low areas along furrows. When the collected rainfall exceeds the storage capacity of soil,that is, when soil becomes saturated, seepage flows through ridges and then leads to the collapse of the contour ridge system. In this study, we conducted rainfall simulation experiments to evaluate effects of rowgrade on nitrogen and phosphorus losses in runoff for brown soil during different erosion stages (seepageerosion, inter-rill and rill erosion) under seepage condition, using the runoff box by which row grade and field slope can be adjusted simultaneously. Results showed that:1) During seepage erosion stage, NO3-N,NH4-N and PO4-P losses under different row grades were in the order of 2° > 10° > 6°, while differences in their concentrations ranked in the sequence of 10° >6° >2°; for inter-rill and rill erosion stages, thesequence of NO3-N loss under different row grades was 10° >2° >6°, while the order of NH4-N and PO4-P losses among row grades was 6° >10° >2° and 2° >6° >10°, respectively. 2) For seepage erosionstage, difference in runoff was the dominant factor that induced the variation of nutrient loss among row grades, while for inter-rill stage it was concentration. 3) The variation of row grade only altered NH4-N lossmechanism during inter-rill and rill erosion stages. These findings in our study would provide scientific guidance for evaluating soil conservation benefit and for implementing the contour ridge system.