We studied the grating-coupling phenomena between surface plasmons and electromagnetic waves in the microwave spectrain. We first present the experimental and theoretical results of ail enhanced microwave transmission though a subwavelength circular annular aperture with and without metallic gratings. We demonstrate that a 145-fold enhancement factor could be obtained with a subwavelength circular annular aperture that was surrounded by periodic metallic gratings. This was assisted by the guided made of the coaxial waveguide and by coupling to the surface plasmons. We present the angular transmission distributions from circular annular apertures, and circular annular apertures surrounded by concentric periodic grooves. At the surface made resonance frequency, the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by consentric periodic grooves have a strong angular confinement with all angular divergence of +/- 3 degrees. We demonstrate that only the output surface is responsible for the beaming effect. Furthermore, we present the field distributions and showed that there is no beaming effect at the off-resonance frequency. (c) 2006 Wiley Periodicals.