A simple and low-crosstalk 1 × 4 silicon mode (de)multiplexer based on multimode grating-assisted-couplers is proposed. Mode transitions can be flexibly controlled by designing the grating period at the phase-matching condition. Due to the contra-directional coupling, precise control of the coupling strength and the coupling length are not needed in the system. Calculation results show that the insertion loss and the 3 dB bandwidths of the device are 0.2 dB and 3.7 nm, 0.34 dB and 7.6 nm, and 0.21 dB and 11.8 nm for the channels which (de)multiplex to the 1st, 2nd, and 3rd modes of the bus waveguide, respectively.