Mechanically-induced wood welding, the linear vibration welding and the rotational dowel welding, is shown here to enhance the water resistance of wood joints with some natural, non-toxic, environmental -friendly additives for outdoor structural application. The motive of wood welding with some additives is shown to be due mostly to form a protection by the melting additives and some amorphous, cells-interconnecting polymer material in the structure of wood. One of additives namely tannin is acetylated by three different processes. The acetylation is identified by CP-MAS 13C-NMR and MALDI-TOF, however the melting point of acetylated increase. The temperature-induced of high-speed rotation is shown here to not be capable of softening acetylated tannins, furthermore Microscopy of SEM confirms that some acetylated tannin can?t be melted. Small amounts of rosin are perfect for improving the water resistance of wood welding joint. Some relative parameters of wood welding, which is important for dry/wet-strength of joint, should be emended according to sample. The products of LVW are obtained with some fixtures.