A large quasi-TEM waveguide for EMC testing is successfully designed by a waveguide with two artificial magnetic conductor (AMC) side-walls comprising an array of a dipole frequency selective surface (FSS) printed on a grounded dielectric slab. In this paper, characteristics of the large quasi-TEM waveguide which dimensions are equal to or larger than the wavelength are investigated. At the resonant frequency of the AMC side-walls, the phase velocity is equal to the speed of light and the electric field distribution is uniform in the large quasi-TEM waveguide. Therefore, a quasi-TEM mode has been achieved in the large quasi-TEM waveguide. In addition, by adjusting the size of the AMC structure, a large quasi-TEM waveguide can be applied to arbitrary frequency for EMC testing.