Abstract: The smart windows based on polymer stabilized liquid crystal (PSLC) are prepared by photopolymerization induced phase separation. The smart windows are transparent at off state (without applied electric field), and turn into an opaque state on application of an external electric field (on state). In this study, the effects of cell gap on the electro-optical properties of smart windows based on PSLC are investigated by UV-visible spectrophotometer. Our result shows that the transmittance at the OFF state decreases with the cell gap increasing, and the threshold voltage and the saturation voltage increase. When cell gap is 5m, the electro-optical properties of smart windows based on PSLC are fine. The transmittance is up to 97% without electric field, but decreases to less than 10% under 40V of applied voltage. Furthermore, the smart window exhibits an excellent response speed, and the switching on time (from off state to on state) is 0.025s, and the switching off time (from on state to off state) is 0.06s. These superior performance makes the smart window enormous potential used as the light management inside buildings.