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 thicknessddepends on the pressurePin the expressionP1/2d=K0for almost all of the discharges examined. It was also pointed out that the constantK0value 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.