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This letter presents a design for diminishing the polarization-dependency of a Si nanowire-based multimode interference (MMI) coupler by using a tapered MMI section. The entrance width and waist width of the tapered MMI section are optimized to obtain a polarization-insensitive MMI length for N-fold self-imaging. The MMI section with a diminished polarization-dependency has a relatively large entrance width, which is beneficial to the separation of adjacent access waveguides for minimizing the coupling between them. One can tune the power splitting ratio of a 2 times 2 Si-nanowire MMI coupler by adjusting the entrance and the waist width of the tapered MMI section.