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Recently, a processing technique resembling plastic-type forming has been developed for solid wood. To improve the recyclability of the formed products, thermoplastics were used as a binder. In this study, a hydrophobic monomer of methylmethacrylate (MMA) was used as a thermoplastic binder and was impregnated into wood, then polymerised by heat to form polymethylmethacrylate (PMMA). The effects of wood acetylation, a hydrophobising treatment of wood, on the extrudability of solid wood impregnated with PMMA (wood-PMMA composite) during extrusion were investigated. The acetylated wood was found to swell much more after the PMMA treatment than the untreated wood. The extrusion loads obtained from capillary fluidity tests of the acetylated wood-PMMA composite were lower than those of untreated composite. The fluidity improvement by acetylation can be attributed to a weakening of the cohesive interactions in the wood polymer, and this is primarily caused by the acetylation reaction itself. When the acetylated wood-PMMA composite was repeatedly extruded, the starting load of the extrusion decreased with increasing extrusion repetitions, probably as a result of decreasing wood particle size.