As a naturally regenerated and biodegradable material, wood and wood composites will be deteriorated in nature. Different poisonous materials, called wood preservatives, are therefore forced into the texture of wood and wood composites to prevent the attack of biological deteriorating agents. Moreover, their hygroscopic property causes absorption of water and water droplets, and even the vapor in the air by the hydroxyl groups in the cell-wall polymers; this process of water absorption would eventually result in dimensional instability and deformation of shape. Water absorption and thickness swelling of wood composites, as their main shortcomings, are also closely correlated to their permeation towards the transfer of different liquids and gases. The susceptibility of woody materials against fire hazards also causes immense losses to both life and property, making it necessary to impregnate wood with fire-retardants. The porous structure and permeability of solid wood species is also practically important for drying wood. Improvement in the permeability of wood, as a porous media, is therefore vital to improve its biological resistance against wood deteriorating agents, consequently increasing its service life, to improve its dimensional stability, and to increase its fireretardancy.