The present invention provides a method of staining wood using iron nanoparticles, characterized in that it comprises the following steps: - iron oxide nanoparticles are prepared by precipitation of iron(II) and/or iron(III) ions in basic medium, wherein the base is a non-ammonium base, to form iron oxide nanoparticles having a Z-average particle size in the range of 1 to 500 nm, as measured by dynamic light scattering method (DLS), and - wood, preferably tannin-containing wood, is impregnated with a composition containing said iron oxide nanoparticles in concentration within the range of 0.1 g/l to 100 g/l. The method results in improved UV resistance properties of the stained wood. Furthermore, method of determination of phenolic compounds content is provided, using the staining method.