Erosion of soils in river, lake, and seabeds is an important component for scour estimation and design of underwater structures. This is because the scour can cause severe structural damage to underwater foundations or embankments. The erosion function apparatus (EFA) method is widely used to estimate the erosion rate of soils in the laboratory, where a soil protrusion of 1 mm thick is exposed to water flow and the time taken to erode this protrusion is measured. However, determining this erosion time is a difficult task because it is only visually inspected, and this can cause considerable measurement errors. Therefore, this study explored the feasibility of using an ultrasonic P-wave reflection monitoring method to more quantitatively assess the erosion rate that otherwise has been measured by visual inspection. The erosion rates were monitored using ultrasonic transducers mounted above a soil surface during the EFA testing on the prepared soil samples containing different clay fractions. Via the P-wave monitoring results, several important semi-quantitative observations were made: an increase in erosion resistance with an increase in the clay fraction, a discontinuous erosion behavior of fine-grained soils with sudden removal of soil lumps by water flows, a continuous erosive action of coarse-grained soils, and inherent heterogeneous erosion even at a specimen scale (i.e., the scale of milli-to-centimeter). While both the P-wave monitoring method and the visual inspection showed similar estimation on the erosion rate, the former was found to provide overall better quantitative assessment, particularly in conditions of very slow or rapid erosion and in the conditions with high turbidity water, unevenly eroded sample surfaces, or limited control on the soil protruding thickness.