The objective of this study is to evaluate fatigue crack propagating behavior in surface modification layer treated by peening. A load-controlled plate bending fatigue tester has been developed in this study. This makes possible to evaluate relation between stress intensity factor range and fatigue crack growth rate, i.e., da/dN-ΔK diagram, in the surface modification layer, since the tester keeps the load constant differently from a displacement-controlled tester. Although the displacement-controlled fatigue tester which keeps displacement constant has been widely used to evaluate the fatigue crack propagating behavior, an applied load, i.e., stress ratio, changes along with crack propagating. In this study, the fatigue crack propagation test was conducted on specimens treated by cavitation peening, which is one of the surface modification techniques, and mechanical properties were also evaluated, including residual stress, yield stress and Vickers hardness, so as to reveal the effect of such mechanical properties varied by the peeing on the fatigue crack propagating behavior. It was concluded that the da/dN-ΔK diagram determined by the tester follows the Paris law and ΔK at da/dN = 10^-9 ～ 10^-8 was increased more than twice by cavitation peening. The increment has a relation with the mechanical properties varied by the peening.