A three-dimensional (3D) finite-element mode-propagation analysis is presented to give an accurate and efficient simulation for silicon-nanowire-based multimode interference (MMI) couplers. For the presented numerical method, a finite-element method mode solver is used to calculate the mode profiles and the propagation constants in a multimode section. The comparison between the presented method and some other numerical methods [e.g., finite-difference time-domain (FDTD) method, beam propagation methods, etc.] is also shown. It is shown that the presented numerical method gives simulation results very close to those from the 3D FDTD (with a deviation less than 1%). Therefore, it is very convenient for the design of Si-nanowire-based MMI couplers.