Soil erosion results in soil degradation, pollutes water resource, blocks riverways and ditches, and imperils eco-environmental construction in Northwest China, especially in the gully region of the Loess Plateau.Re-vegetation is an effective method to reduce soil and water loss and improve eco-environment in the Loess Plateau.Therefore,the impacts of re-vegetation on water cycle have been one of the key issues in eco-environmental study in the loess region.Using a paired watershed approach,the impacts of re -vegetation on water cycle was studied in a watershed on the Loess Plateau during the period of 1956 to 2000.Deciduous trees,including locust (locusta L.),apricot (praecox L.)and elm (ulmus L.),were planted on a watershed with treated area exceeding 80% ,while a natural grassland watershed remained unchanged.The cumulative runoff yield in the treated watershed was reduced by 37% as a result of afforestation in 44 years.During the first 15-20 years,annual runoff reduction increased with the increase of the age of the trees planted and it didn't change much during the next 24-30 years. Reduction in monthly runoff occurred mainly from June to September.Compared with the natural grassland watershed,cumulative reduction of volumetric water content was about 222mm as a result of afforestation at a rate of 5mm per year.The cumulative evapotranspiration in treated watershed comparatively increased by 620mm at a rate of 14mm per year,which will cause soil desiccation.