Cellulosic substances in polymer matrices are immiscible and non-dispersive because of mismatch between their surface properties. In order to fabricate a natural fiber-reinforced plastic, hydrophilic cellulosic substances should be modified to hydrophobicity to make it miscible and dispersible to polymer matrices. Herein, this work shows potential application of vapor-phase-assisted surface treatment as a new modification approach for cellulosic substances. Cellulosic substances were modified to improve the compatibility in the polymer matrix. The modified cellulose substrates were characterized by Fourier transform infrared spectrometer spectroscopy, thermogravimetric analysis, and scanning electron microscope, and the resulting shows the grafting polymer on cellulosic substances indeed. To more enhance compatibility in polymer matrix, grafting of the methacryloyl groups on cellulose substance was prepared through gaseous methacrylic anhydride on cellulose surface, and then higher modified cellulose substance was carried out with vapor-phase graft polymerization of methyl methacrylate finally. The vapor-phase-assisted surface treatments could highly modify the cellulose surface compared to conventional method in liquid process.