A reconfigurable two-mode mux/demux device in planar waveguide was proposed. The simulated mux/demux extinction ratio was ≥ 35 dB with ≤ 0.4 dB excess loss. The device was realized in polymer materials using the thermo-optic effect. It was characterized via a tunable laser source at 1550 nm. Its mux/demux performance in both routes was demonstrated and compared with the theoretical prediction. The device is easy to implement and has applications in future multimode optical communication systems to further extend transmission capacity.