The heat generated by a thin film heater causes the temperature of a nearby waveguide to increase. The thermal coupling increases with the waveguide depth. Thermal coupling could be reduced by etching a trench between the waveguides. A rib structure under the trench between the waveguides was proposed to further reduce the thermal coupling. The temperature profiles of thermo-optic waveguides are analyzed by the finite element method and show significant improvement on reducing thermal coupling between the waveguides.