Hemicelluloses as matrix substances showed an important role in nanofibrillation of wood pulp. Never-dried and once-dried pulps with different amounts of hemicelluloses were fibrillated using a grinding treatment. The degree of fibrillation was evaluated by scanning electron microscopy observation of the fibrillated pulps and light transmittance measurements of the fibrillated pulp/acrylic resin composites. With a one-pass grinding treatment, the once-dried pulp with higher hemicellulose content was fibrillated into 10-20 nm wide fibers as easily as the never-dried pulps, while the once-dried pulp with lower hemicellulose content was not fibrillated into uniform nanosized fibers. This result indicates that hemicelluloses act as inhibitors of the coalescence of microfibrils during drying and facilitate the nanofibrillation of once-dried pulp. Furthermore, hemicelluloses provide adhesion between nanofibers, contributing to reduction of thermal expansion and enhancement of mechanical properties in the composites.