Based on the long-term observation and from the viewpoints of water balance and runoff-and sediment generation, this paper studied the generation processes of runoff and sediment on two typical woodlands, artificial P. tabulaeformis and secondary natural P. dadidiana, and uncultivated slope-land in Loess Plateau under hypo-rainfall. The results showed that within the range of 5.0 - 50.0 mm rainfall, the total interception of canopy and litter was 15.45 % - 56.80 % for P. tabulaeformis and 20.56% - 47.81% for P. dadidiana, and decreased with increasing rainfall. Woodlands had a higher soil water infiltration capacity than uncultivated slope-land, especially in 0-20 cm soil layer. Both the two woodlands did not generate runoff under regular rainfall. Under the assumed rainfall of 2.5 mm x min(-1) intensity and 30 min duration, P. dadidiana stand did not produce runoff, but the runoff velocity and sediment-carrying capacity of uncultivated slope-land were 23.5 times, and runoff shearing stress and energy were 8 times as much as P. tabulaeformis stand. The runoff-and sediment generation on P. tabulaeformis stand decreased by 87.6% and 99.4%, respectively, compared with those on uncultivated slopeland, which was well accorded with the average observed value in runoff plots during 1988 - 2000. The theoretical analysis on the generation mechanism of woodland runoff and sediment may be effective to evaluate the benefits of forest in soil and water conservation.