An ultra-compact 1310/1550 nm wavelength division (de)multiplexer based on a channel-shaped multimode interference structure was proposed and fabricated on an InP platform. The device has been simulated and optimized with a low insertion loss of 0.1 dB at 1310 nm wavelength and 0.33 dB at 1550 nm wavelength. The device features a notably compact footprint with an MMI region just 48 μm in length. Measurements revealed extinction ratios of 7.1 dB at 1310 nm and 5.9 dB at 1550 nm, accompanied by insertion losses of 7.07 dB and 3.03 dB for these wavelengths.