In this study, we made a systematic investigation in the enhanced transmission of X-band microwaves through a single subwavelength slit aperture surrounded by periodic grooves in metallic aluminum plates. The influence of the grooves’ number, width, and depth on the transmission spectra was examined. By tuning these geometrical parameters, the transmission enhancement could be maximally optimized. The transmission spectra at s-polarization was also measured and compared to those at p-polarization. It is shown that the extraordinarily enhanced transmission at p-polarization is associated with the surface waves of Brewster–Zenneck modes on the metal-air interface. Our experimental results will be helpful to the control of the propagation of electromagnetic waves in the microwave regime.