Using a simple model, it is shown that because of the relativistic electron-mass variation, the mode-converted electrostatic electric field exhibits hysteresis in the vicinity of critical layer. This hysteretic effect is to increase the resonance absorption of obliquely incident p-pola-rized light in a way by increasing the large amplitude region of the electrostatic field. When the threshold light intensity for the occurance of hysteresis is exceeded, the fractional absorption increases drastically above the linear results.