Abstract: A wood modifier composition containing a mixture of a water-soluble, low molecular weight chitosan substance and colloidal silica is disclosed. The mixture contains from about 15 to about 300 parts by weight of SiO.sub.2 per part of the low molecular weight chitosan substance, and may also contain one or more compounds capable of improving wood quality, selected from boron, copper, silver, aluminum, titanium, manganese and black lead. The wood modifier composition is preferably used as an aqueous solution having a pH of about 5 to about 12. The wood modifier composition according to the present invention can be used as a neutral aqueous solution having a low viscosity which can easily penetrate into wood cells and be safely fixed therein.