A new version of thermo-optical modulator has been presented here. The device is a pair of single-mode parallel channel waveguides on glass substrate by using K-ion exchanging technique. A Ti-film strip heater is set on the separation to control the coupling coefficient between the two parallel waveguides. The deduced expression shows that the change of coupling coefficient is approximately proportional to the change of temperature. The output of coupler varies with sine square in which phase angle is linearly related to change of the index. The phase angle is controlled by thermo-optical effect in our study. The device is designed to operate at wavelength 632.8 nm, the measured modulation depth of the device is as high as 90%.