In this letter, the mode demultiplexers for rectangular multimode waveguides are proposed based on a spatial sampling and the multimode interference (MMI) effect. The modes of the rectangular waveguide are sampled by an equally spaced waveguide array and become the supermodes of the array. The supermodes are passed to a MMI coupler with phase shifters at the input ports. Owing to the MMI effect, the output optical field will concentrate on the corresponding output port, whose port number coincides with the order of the mode inside the rectangular waveguide.