The modeling of dielectric properties of wood–polymer composite, considered as a multicomponent layered system, was performed. The obtained solutions were used to determine temperature dependences of real and imaginary parts of complex dielectric permittivity characterizing the wood component in the natural and polystyrene-modified wood in the directions parallel and perpendicular to the fibers. The analysis of the calculated dependences proved that the structural changes of wood depended on the applied method of modification. The main effect attributed to the styrene polymerization occurring in wood placed in a hydrophilic heating medium was an increase in the ordering degree of the mesomorphous regions of cellulose. As a consequence, a part of the polar, functional groups distinguished by particularly high activation energies (i.e., low probability of a dipole jump to an adjacent site) was eliminated from participation in the relaxation processes. The application of a hydrophobic heating medium during modification procedure resulted in a permanent swelling effect of the wood component of the composite. Within the applied investigation method it was found that the changes in increment of dielectric permittivity referring to the β-relaxation process in wood substance can be the specific criterion of structural changes in wood modified with polystyrene. The distribution functions of relaxation times for natural and modified wood substance were determined.