Epitaxial yttrium-iron garnet (YIG) films with various thicknesses were fabricated on gadolinium-gallium garnet substrates by a metal–organic decomposition (MOD) method, and their dynamic and static magnetic properties were investigated. We varied the thickness of the YIG films by repeating spin-coating of the MOD solution and pre-annealing. The lattice constant and the crystal size of the YIG films were almost independent of the thickness (the coating count N). The damping factor and coercive field increased with N. Moreover, the saturation magnetization MS measured using a vibrating sample magnetometer increased with N, although that measured by ferromagnetic resonance was almost independent of N. Thus, both dynamic and static magnetic properties for the YIG films obviously degraded with increasing N. Judging from these results, the repetition of the coating process generates other ferromagnetic micro-crystalline phases at grain boundaries, for instance, γ-Fe2O3 and Fe3O4, which could cause degradation of the magnetic properties of the YIG films prepared by the MOD method.