This paper discusses the relationships between the evolution of valley side slopes of hilly lands such as Soya Hill and Shiranuka Hill in Hokkaido and rock properties such as cone penetration hardness, point-load tensile strength, Schmidt rebound and infiltration capacity. The bed rocks of the areas studied are classified into five groups in terms of combinations of mechanical strength (S) and infiltration capacity (Ic). The classifications are as follows; (1)WH group (weak S, high Ic), (2)WL group (weak S, low Ic), (3)ML group (medium S, low Ic), (4)SH group (strong S, high Ic), ESL group (strong S, low Ic). The V-shaped valleys examined are assumed to extend their length by headward erosion. Hence, the distance from each valley head to the sites where morphometry was performed; X (m), implies the time elapsed since the beginning of erosion at the site. Thus, the relationship between the mean angle of a valley side slope (θ) and X is inferred to indicate the temporal evolution of the valley side slope. The evolution of the slopes occurs as follows; θ increases in SH group and SL group, is constant in ML group, and decreases in WH group and WL group. However, after the lapse of a certain amount of time, θ is constant in all of the groups. This value of θ is in accordance with the magnitude of mechanical strength. Thus, the rate of the evolution of a valley side slope is deduced to be mainly affected by mechanical strength and not by infiltration capacity. In a hilly region in an early stage of erosion, the evolution of valley side slopes shows the same changes as the case of coastal terraces.