The heating process of frozen wood exposed to plane microwaves is numerically investigated. The nonlinear heat conduction problem involving phase change such as wood freezing is solved by a specific 3D volumetric enthalpy-based finite element method. Dielectric and thermal properties are a function of temperature and moisture content. The numerical model is validated by experimental and analytical means. As an application, we studied the effect of moisture content and frequency on microwave heating of trembling aspen wood.