The electromagnetic wave enhanced transmission (ET) through the sub-wavelength aperture was an unconventional physical phenomenon with great application potential.It was important to find a general design method which can real-ize efficient ET for arbitrary-shaped apertures.For achieving ET with maximum efficiency at specific frequency through arbitrary-shaped subwavelength aperture,a topology optimization method for designing metamaterials (MTM) microstruc-ture was proposed in this study.The MTM was employed and inserted vertically in the aperture.The description function for the arbitrary shape of the aperture was established.The optimization model was founded to search the optimal MTM microstructure for maximum enhanced power transmission through the aperture at the demanded frequency.Several MTM microstructures for ET through the apertures with different shapes at the demanded frequency were designed as exam-ples.The simulation and experimental results validate the feasibility of the method.The regularity of the optimal ET microstructures and their advantages over the existing configurations were discussed.