【Objective】Soil aggregate is the basic unit of soil structure. Soil aggregates in runoff cause severe abrasion of soil surface, thus significantly affecting water infiltration rate of the soil surface, sediment transport and deposition, and development of soil erosion. With the aggregates breaking into numerous small particles, soil erosion starts. Researches have been reported illustrating that changes in roughness of the surface of a slope directly affect hydraulic characteristics of overland runoffs on the slope, like friction factor,flow depth and so on, which are the best hydraulic parameters to characterize aggregate erosion in the process of aggregate transport with runoff on the slope. Therefore, roughness of the surface of a slope may also affect disintegration of soil aggregates in runoff to a varying extent relative to degree of roughness. 【Method】In order to study erosive effect of soil aggregates in runoff on slopes different in surface roughness, an experiment was conducted in the artificial rainfall simulation lab of the Huazhong Agricultural University. The soil used in the experiment was Ultisols, collected from Xianning of Huibei Province, China. The soil sample was air-dried and oven-dried at 40℃ for 24 h to ensure that the soil sample was constant in moisture content. Before the experiment, portions of soil aggregates 20 g each were weighed out, 0.0001g in diversion, and were put separately in flasks containing ethanol (95%) for 10 min to expel air from the aggregates so as to eliminate the risk of breaking up the aggregates in the process of fast-wetting. Stainless steel flumes, adjustable in slope gradient, were set up. The pretreated soil samples were spread out in the flumes and prepared into 5 degrees of roughness (d=0, 0.25~1, 1~3, 3~5 and 5~7 mm) separately. The flumes were adjusted to 10 ° in gradient and the water flow controlled at 0.4 L s^-1.The experiment was designed to explore abrasion effects of soil aggregates in runoff as affected by particle size of the aggregates (7~5, 5~3 and 3~1 mm), roughness of the surface (d=0, 0.25~1, 1~3, 3~5 and 5~7 mm) and transport distance (9, 18, 36, 54, 72 and 108 m) and effects of particle size, surface roughness, transport distance and some related hydraulic parameters (flow depth, friction factor, flow shear stress) on aggregate abrasion. 【Result】Results show that (1) aggregate abrasion aggravated with rising particle size of the aggregates, and with rising surface roughness, too, when transport distance remained the same. Yet, the effect of the latter was more obvious than that of the former. The effects of roughness and interaction of roughness and particle size all reached the significance level p<0.05, regardless of transport distance, however, the effect of particle size was not significant when the transport distance reached 54 m or 72 m. (2) On slopes the same in roughness, abrasion rate followed a paracurve, rising and then falling with increasing transport distance. Regardless of transport distance, the abrasion rate of the aggregates, 7~5 mm in particle size, increased with increasing roughness; but that of the aggregates, 5~3 mm or 3~1 mm in particle size, varied irregularily with increasing roughness. (3) When the transport distance was fixed to be 36m, W_r/W _iof aggregates declined as a power function of increasing depth of the runoff, regardless of roughness, which may be attributed to the joint effect of roughness and runoff depth, and the effect of roughness was more obvious in the case of the flow rate and slope gradient set in the experiment. Aggregate abrasion rate decreased as a power function of increasing friction factor and/or flow shear stress, declining first and then leveling off. However, in the case of varying roughness, the effect of runoff depth on abrasion rate of aggregates as affected by slope gradient and flow rate needs to be studied further. 【Conclusion】All the findings in this experiment may serve as reference for researchers in studying mechanisms of red soil aggregates breaking down in runoff and have some great theoretical significance to the development of proper soil erosion models.