Refractive index of polymethyl methacrylate (PMMA) and polystyrene decreases with increase of temperature and due to variation of refractive index the propagation constant of PMMA and polystyrene thin films change. Using this property of those polymer thin films a simple thermal sensor is demonstrated where a channel waveguide to polymer thin-film coupler is used. A thin film of PMMA is deposited on the channel waveguide to polymer thin-film coupler is used. A thin film of PMMA is deposited on the channel waveguide by spin coating method. Optical power from an LED of wavelength (lambda) equals 1.3 micrometers is coupled to the channel waveguide and the throughput power of the channel waveguide is detected by a p-i-n diode at different temperatures. Variation of the throughput channel power is correlated with the change in temperature for potential use in thermal sensing and switching applications. A thin film of polystyrene is also deposited on another channel waveguide and same experiment is performed. It shows the increase in range of temperature for measurement.