Grating-coupling phenomena between surface plasmons and electromagnetic waves are studied in the microwave spectrum using metallic gratings. Transmission and reflection measurements were carried out to observe the transmitted and reflected radiation around the surface plasmon resonance frequencies. Grating structures with subwavelength apertures were designed for transmission experiments. Measurements were done in the microwave spectrum of 10−37.5 GHz, corresponding to a wavelength region of 8−30 mm. The Al samples had a grating periodicity of 16 mm. A 2 mm-wide subwavelength slit was opened for transmission samples. Samples with one/double-sided gratings displayed remarkably enhanced transmission and directivity with respect to the reference sample without gratings. The experimental results agreed well with theoretical simulations. ~50% transmission at 20.7 mm, ~25-fold enhancement, and ±4° angular divergence was achieved with a ~λ/10 aperture. Reflection measurements of one-sided grating sample resulted in clear observation of surface plasmon coupling phenomena. Reflectivity peaks were measured around theoretically calculated resonance frequencies.