Thermo-optic bistability at room temperature has been obtained in CdS films (thinckness 1～3 μm) excited by Ar+-laser (A=514.5 nm). The transverse effects and on/off waves was recorded in the thermo-optic bistability. Different behaviours of CdS crystals for the thermo-optic bistability and the thermo-optic breakdown was investigated. Minimum distance (200 μm) between the OB elements that works separately has been calculated.