In this doctoral thesis the influence of styrene monomer modification on the physical and mechanical properties, viscoelastic and thermal properties and morphological structure of fir wood was examined. In addition to the characterization of the modified material, physical and mechanical properties (bending strength and modulus of elasticity) and basic chemical composition of fir wood (cellulose, lignin, extractives) were determined alongside the ash content in unmodified fir wood samples from Zalesina region. The analysis of the average annual ring width and the analysis of the raw wood mositure content (as important factor influencing the lamellae sawing process) were also conducted. Preliminary tosylation of wood cell walls and free radical polymerization under mild polymerization conditions were performed by grafting polystyrene chains to OH groups of fir cellulose chains. The primary viscoelastic properties of unmodified and modified fir wood were examined by dynamic -mechanical analysis (DMA) where the presence of polystyrene in the cell walls was manifested as an 10,39% increase in the storage modulus. The confirmation of the modification, i.e. the presence of polystyrene, was done using the infrared (IR) spectroscopy and thermogravimetric analysis (TGA). The morphology of the fir wood was determined by SEM analysis where the presence of polystyrene in fir tracheides (polymerized styrene in cell lumens) was visually confirmed. Examination of the swelling of the samples revealed a significant improvement in the dimensional stability with a decrease in swelling in the longitudinal direction by 91.18%, radial direction by 81.42%, tangential direction by 74.61% and volume swelling by 78.06%. The results of the surface energy and contact angle determination confirmed the high hydrophobicity of the surface of fir wood after modification as an evidence of polystyrene presence not only in the cell walls and cell lumens but also on the surfaces of the modified fir wood samples.