A novel temperature optical sensor configuration has been realized using standard microelectronic technology. The sensor is based on mode mixing principle. The sensing element consists of a singlemode all-silicon waveguide, followed by a two-modes section and an output Y branch to separate the two output channels. The interference between these modes produces a periodically repeated optical intensity distribution along the propagation axis. A light steering from one output channel to the other caused by the temperature change of the structure can be observed. The thermo-optic effect, in fact, implies a variation in the effective refractive indices of propagating modes, and consequently, a phase shift between the modes themselves. In this way, a continuous variation of the power, as a function of the temperature, can be observed on the two output waveguides.