Chemical modification of wood is a potential way to obtain high quality wood. In this study, methylolurea was used to modify the polar wood by in situ polymerization. The mechanical properties and dimension stability of the wood-methylourea composites were investigated, and the modified samples were also characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The mechanism of in situ polymerization was presented in graphs. The test results showed that the mechanical properties and dimensional stability of natural wood were improved remarkably. FTIR analysis suggested that the methylolurea polymerized with the active groups of wood cell wall. XRD test showed that the crystallinity of wood increased after modification. Finally, the SEM analysis proved that the good interfacial adhesion of wood modifier between wood fiber and polymer. (C) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 933-938, 2013