Soil and water conservation benefits of different afforested trees in the loess hilly-gullied region were evaluated based on runoff and soil erosion measured in vegetated runoff plots between 1993 and 2002. Soil water conditions and their dynamics under forestation trees were also analyzed according to soil water content measured in 2002. Results showed that runoff and sediment were correlated with rainfall. Multiple regression analysis indicated that there was a positive correlation between runoff and the product of rainfall depth and maximum 0.5-hour rainfall intensity at the significant level of 0.01, as well as between sediment and the product of rainfall depth and maximum 0.5-hour rainfall intensity. However, both runoff and sediment were negatively correlated with vegetation cover. The multiple correlation coefficient ranged from 0.465 to 0723 for runoff and was 0.253 for sediment. In addition, soil and water conservation benefits were reasonably evident for all afforested trees. Compared with farmland, runoff and sediment decreased to 4.8%-52.9% and 26.8%-860%, respectively. The soil and water conservation benefits of Hippophae rhamnoides and its mixtures were greater than those of Pinus tabulaeformis whose cover was poor in the early growth years, during which runoff and sediment were even greater than those of farmland. But the benefits increased as the trees grew. However, the soil and water conservation benefits of Hippophae rhamnoides and its mixtures were good even in the early growing years and became even better as trees grew over time in eleven years. Soil water content below 30 cm soil layer under the Hippophae rhamnoides and its mixtures decreased during the entire growing season, being highest in April and lowest in October. Water consumption depths of the Hippophae rhamnoides under monoculture and biculture were 220 and 300 cm, respectively, in 2002. There were significant differences in soil water content for mono-species of Hippophae rhamnoides and its binary mixtures during the entire growing season. The tree species, in terms of soil water content, ranked in the ascendant order of Hippophae rhamnoides-Populus simonii Carr mixture, Hippophae rhamnoides, and Hippophae rhamnoides-Pinus tabulaeformis mixture.