A Cd x Zn 1-x Te/ZnTe multiple quantum well (MQW) optical bistable device has been fabricated, and its optical bistability is investigated at room temperature. The result indicates that the threshold and contrast ratio for the optical bistability in Cd x Zn 1-x Te/ZnTe MQW optical bistable device are about 363 kW/cm 2 and 4:1, respectively. On the basis of the excitonic nonlinear theories, excitonic absorption spectrum in the Cd x Zn 1-x Te/ZnTe MQW, we attribute the major nonlinear mechanism for the optical bistability to the saturating effect of excitonic absorption.