This paper discribes the characteristics and approximation of a radial turbine for exhaust gas turbochargers under steady and unsteady flow conditions. The experiments were carried out at several velocity amplitudes and pulse frequency ranges with sinusoidal waveforms. The approximation was conducted on the assumption of a one-dimensional equivalent nozzle considering the ratio of velocity amplitude. As a result, the following is clarified. For most turbines under unsteady flow conditions, the time-mean mass flow rate becomes less than the steady one at the same turbine expansion ratios as the ratio of velocity amplitude becomes large. Approximated results of mass flow rate using the equivalent nozzle show good agreement in the characteristics of both steady and unsteady flow conditions in consideration of windage states.