Optimal transmission characteristics of THz wave through a slit aperture with parallel grooves on both sides are analyzed numerically by the finite element method. Simulations results show that the center transmissivity can be up to above 95% because of the enhanced transmission effect induced by surface plasmon polaritons. With the increase of periodicity length p or center hole width d or groove depth s, the center frequency shifts red linearly, the center transmissivity increases firstly and then decreases. Furthermore, with the increase of d/p, w/p, or s/t, the center transmissivity increases firstly and then decreases, which the optimal values d/p= 0.25–0.3, w/p= 0.5, and s/t= 0.2. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:559–561, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27366