The radio-frequency glow discharges of several kinds of gases were examined to measure the ion sheath thickness at the cathode electrode. At intermediate gas pressures around 0.05– 0.5 Torr, the sheath thickness d depends on the pressure P in the expression P1/2d=K0 for almost all of the discharges examined. It was also pointed out that the constant K0 value decreased linearly against a mass of the predominant ion in the plasma. The discrepancy between the sheath thickness measured in this work and by theoretical solution was discussed for the argon discharge.