We propose a method to improve the performance of N × N(N⩾3) multimode interference couplers (MMIs) by suppressing propagation constant errors through controlling excited guided modes and the width of MMI, without increasing their fabrication complexity. 5 × 5 MMIs were fabricated on a commercial 220-nm silicon-on-insulator (SOI) platform to verify the method. Measurement results show that insertion losses (ILs) and imbalances (Ims) are reduced from 2.28 dB to 0.16 dB and from 6.94 dB to 0.33 dB, respectively.