Soil erosion is one of the most environmental problems in arid areas of the Loess Plateau in North China. And understanding the relationships between vegetation covers, especially man-made forests, and runoff and soil erosion is key to ecological restoration in the region. Previous studies showed that forests can reduce runoff amount and protect soil from eroding in loess gulley area of the Loess Plateau in North China. But the man-made forests such as Robinia pseudoacacia plantations would consume more water and may lead to severe loss of soil water, which is harmful to the hydrologic functions of forests in the Loess Plateau. We examined the hydrologic differences of two watersheds with different landuse and landcover types, one natural forest and one artificial forest in the Caijiachuan watershed in the loess hilly region of western Shanxi Province. These two neighboring experimental watersheds (one of which was planted artificial trees such as Robinia, Pinus and Platycladus, and the other has been closed for 26 years) have the similar characters of soil、geology and physiognomy. Based on water observation data and precipitation data collected from 2004 to 2009 in experimental watersheds of loess areas, comparisons of the annual runoff and the rainy-season runoff between natural forest and artificial forest, and analyses of the correlation and regression under different types of rainfall were used to discuss on the relationships between runoff and vegetation covers. The results in this study showed that the annual runoff and the rainy-season runoff (June-September) from the natural forest watershed was significantly lower than that from the artificial forest watershed, 58.6% and 48.0%, respectively. Annual and rainy season overland flow from the natural forest watershed was lower by 83%,81.1% respectively when compared to that from the artificial forest watershed. Baseflow was negligible in the artificial forest watershed. Under A-type rainfall(storm events with short duration and high intensity), peak discharge per unit area in the natural forest watershed was significantly lower by 83.4%, compared with that in the artificial forest watershed. The types of rainfall had impact on runoff in small watersheds with different vegetation covers. For the artificial forest watershed, the average peak discharge per unit area under A-type rainfall was highest, about 10.1 times of that under B-type rainfall(storm events with long duration and high intensity), about 3.4 times of that under C-type rainfall(storm events with long duration and low intensity). For natural forest watershed, the average peak discharge per unit area under A-type rainfall was also highest, about 6.8 times of that under B-type rainfall, about 1.6 times of that under C-type rainfall. So we concluded that flood events and thus soil erosion would likely to occur under A-type rainfall events. And some measures for water and soil conservation should be taken especially under A-type rainfall events. This study suggested that natural forests could reduced more runoff and peak discharge than the artificial forests. Natural vegetation recovery had better hydrologic benefits in water and soil conservation and thus should be promoted in the loess gulley area of the Loess Plateau. Also, man-made forests should be managed to realize their intended hydrologic functions.