The effects of microtopography on erosion remain uncertain, and contradictory findings have been reported in past decades, hindering full understanding and accurate prediction of the quantity and quality of soil erosion caused by rainfall‐runoff. In this study, we applied a dynamic wave and Hairsine‐Rose model simulator on hillslopes composed of various sizes of mounds and puddles. For the tested soil particles whose settling velocity is 0.0035−300mm/s $0.0035-300\,\mathrm{m}\mathrm{m}/\mathrm{s}$ and rainfall intensity of 100mm/hr $100\,\mathrm{m}\mathrm{m}/\mathrm{h}\mathrm{r}$, we found that soil erosion quantity is significantly affected by microtopographic roughness, even under the same rainfall‐runoff conditions. In addition, we found that soil particle size can potentially determine whether there is a direct or inverse proportionality relationship between microtopographic roughness and soil erosion rate on hillslopes. That is, when the land surface layer is composed of relatively small soil particles, rough microtopographies accelerate soil erosion rates compared to smooth microtopographies. In contrast, when it is composed of large soil particles, soil erosion increases more on smooth microtopographies than on rough microtopographies, at least under the tested conditions. Finally, it was revealed that the progress of the microtopographic scale armoring process is closely related to the microtopographic roughness as well as the grain size. Thus, to appropriately consider the role of microtopography in soil erosion problems, the processes of pickup, advection, and deposition related to armoring must be physically considered. Without sufficient information on these features, low prediction accuracy is expected, not only of the quantity but also in the quality of soil erosion. Key Points Microtopography can change the quantity and quality of watershed erosion even under the same hydrological conditions Whether soil erosion rate is directly or inversely proportional to microtopographic roughness is determined by soil particle size Microtopographic scale armoring processes play an important role in size‐selective soil erosion on a hillslope