The propagation characteristics of millimeter waves in an H guide loaded with yttrium iron garnet films are investigated. A perturbation theory is used for evaluating the dispersion curve. Theoretical results are confirmed by experiments carried out in the millimeter wave frequency range 40–50 GHz using yttrium iron garnet films. Dispersion characteristics for a laminated structure of yttrium iron garnet films are also examined. The gyromagnetic resonance behavior with a higher quality factor is observed when the dielectric slab is loaded on both sides with yttrium iron garnet films.