The formation of radial electric field Er in the tokamak edge region is calculated based on the collisionless ion orbit loss. The ion orbit loss generates a negative Er, which in turn affects the ion loss. As a result, Er can saturate at either a low or a high value, depending on the plasma parameters. When the ion temperature in the plasma edge is higher than a threshold, a self-sustaining growth in both ion loss and Er is found, leading to a high saturation value of Er in milliseconds. This mechanism provides a possible explanation for the formation of a strong radial electric field during the transition from the low (L) to the high (H) confinement regime observed in tokamak edge plasmas.