The frequency dependence of the sustaining voltage in a radio frequency discharge has been investigated under minimum sustaining and constant power conditions in Ar. In addition to the well‐known feature that the sustaining voltage is high at low frequency and low at high frequency, the difference between monoatomic and polyatomic gases is recognized. The phase shift between voltage and current at 1 Torr decreases with increasing frequency above 500 kHz. This results from the electron density modulation having a delay with respect to the applied voltage. The phase shift at high frequency increases with increasing applied voltage, due to the balance of drift and diffusion fluxes of electron. The spatiotemporal net excitation rate of Ar(3p5) was measured at 100 kHz, and 4 MHz, and 13.56 MHz. It is reconfirmed that the sustaining mechanism of the discharge at low frequency is ionization by secondary electrons from the electrode, while at high frequency it is ionization due to reflected electrons by the oscillating sheath. At middle frequency, 4 MHz, the sustaining mechanism drastically varies with input power density from the high‐frequency type to the low‐frequency type.