Abstract: Many studies have shown the effects of mechanical properties on soil erosion. However, most have focused on the strength of bulk soil, with little attention paid to the strength of the aggregates. The objective of this study was to assess the effects of aggregate tensile strength and friability on aggregate breakdown under simulated rainfall. Different size aggregates of Ultisols, derived from Shale (S) and Quaternary red clay (Q), were exposed to simulated rainfall of 60 mm h−1. Fragment size distribution and splash loss were compared. The results showed that tensile strength decreased when aggregate size increased. For 1- to 2-mm aggregates, tensile strength of Q was greater than S, but the opposite was true for 10- to 20-mm aggregates. Estimates of friability from the coefficient of variation method were different with aggregate sizes tested and always gave larger numerical values than the volume dependence method. Aggregate breakdown was closely related to tensile strength because both depend on aggregate microstructure. Good correlations were observed between tensile strength and normalized mean weight diameters under different amounts of cumulative rainfall. As aggregate size increased, mean weight diameter of fragments increased, but normalized mean weight diameter and splash loss decreased. The tendency that aggregate splash decreased with increasing aggregate size was more obvious for the soil with low friability. For all size classes, the aggregates of soil with low friability produced more splash materials. It might be possible, with further experimentation, to develop empirically based criteria for stability of soil to erosion using tensile strength and friability.