A method to evaluate and compare nonlinear property of flood mitigation capacity of watersheds is proposed in this report. First, the observed rainfall-outflow data in the watershed are expressed using a runoff model. Then a rain pulse of R (mm) is applied to the model to ascertain the peak discharge Q_max(mm/h). Ratio τ_R (= R/Q_max), which is designated as the peak discharge relaxation time, is the index of flood mitigation capacity. Values of τ₅₀ and τ₁₀₀ for typical watersheds of Japanese rivers are shown. For analyses of them, the literature describes some constructed tank models.