A compact bi-directional (BiDi) triplexer using grating-assisted multimode interference (MMI) coupler is proposed based on silicon nanowire waveguides.Because of the high index contrast between silicon and silicon dioxide, the size of the structure is greatly reduced with a footprint of 2.5 \times 911 (\mu m). Asymmetrical ports are introduced in the MMI structure to satisfy the bandwidth requirements of the industrial standards ITU-T G.983.3-dB bandwidths of 100, 22, and 15 nm are obtained for the wavelengths of 1 310, 1 490, and 1 550 nm, respectively. The device can be readily fabricated using a commercial CMOS process.