The Loess Plateau is one of the areas that suffer the most serious soil erosion and the soil in the area is characterized by low soil fertility and high soil erodibility. Thanks to its inherent characteristics and physicochemical properties, biochar has become a novel soil structure amendment. However, so far studies on biochar have mostly focused on its effects of improving soil physical and chemical properties and plant growth conditions, reducing greenhouse gas emission and remedying polluted soil, and demonstrated that its positive effects on soil quality, soil bulk density, soil porosity, quantity and structure of soil aggregate and soil water dynamics. Little has been reported on its effects on soil resistance to erosion. Getting to know the effects of biochar on soil erosion will be of great significance not only to soil and water conservancy, but also to soil building and crop yield. An indoor artificial rainfall experiment was carried out to explore effect of biochar on sheet erosion on loessal soil slope. The experiment was designed to have one rainfall intensity (90 mm h^-1), five application rates (0%, 1%, 3%, 5% and 7%) and three particle sizes (<2mm, <1mm and <0.25mm) of biochar of sawdust. Results show that the effect of biochar application on erodibility of loessal soil plow layers is attributed mainly to its effects altering composition and porosity of the soil and the effect of its own properties on water, which were embodied in delayed runoff and reduced runoff and sediment yields and sediment in runoff. Incorporation of biochar, the same in particle size, shortened the duration of runoff and the effect intensified with rising application rate of biochar. However, when the application rate was lower than 3%, it delayed runoff, but when the rate went on rising, it affected runoff reversely. Incorporations of biochar, the same in rate, but different in particle size, did not show much difference in affecting time of runoff yield. Incorporation of biochar of any size at a rate of 1% demonstrated runoff and sediment reducing effects in every rainfall event. So, in exploring effects of biochar application rate on runoff and sediment yields, it is essential to take into account particle size of the biochar. Biochar application rate, biochar particle size, interaction of the two elements and other uncontrollable factors were responsible for 21.35%, 7.27%, 41.19% and 30.20%, respectively, of the variation of runoff yield brought about by rainfall and for 32.9%, 8.66%, 35.21% and 23.23%, respectively, of the variation of sediment content in runoff. As the biochar particle size does not have much influence on soil erodibility and sediment yield, ,biochar application rate, interaction between content and particle size of the biochar, and other uncontrollable factors, are the factors contributing 37.16%, 34.46% and 28.38%, respectively, to variation of soil erodibility and 32.48%, 35.59% and 31.94%, respectively, to variation of erosion. Obviously, biochar application rate and interaction between content and particle size of the biochar are major factors affecting rainfall erosion significantly, and particle size of biochar is a minor one. Though this study is a small scale research on mechanism of biochar affecting soil erosion, the findings in this study may serve as reference of scientific significance to large-scale researches on effect of biochar on soil erosion in future.