An integrated optical thermo-optic switch has been designed using silicon-on-insulator (SOI) waveguide technology. The switch consists of two cascaded waveguide couplers with thermo-optic heaters on the waveguide arms connecting the two couplers. A detailed modelling of the optical and thermal operation of the switch has been made. A single-mode ridge waveguide structure with large cross-section was designed. The fibre-waveguide mode coupling loss was calculated to be 0.6 dB. According to the thermal modelling, a 3 K temperature change is enough to switch the output state of the device. The switching time is expected to be 0.2 ms and the electrical power consumption 120 mW.