Using both experimental and numerical studies, we show that, upon cyclic indentation with a flat cylindrical indenter, the steady-state rate of indentation depth propagation can be well characterized by a simple power-law function of the stress intensity range and the maximum stress intensity, which is qualitatively similar to the steady-state growth of a fatigue crack. The simple law of indentation fatigue has the potential to extract the fatigue properties of materials with small volumes and clarify the fatigue crack mechanisms.