Crystallite refers to a single crystalline grain in crystal aggregates, and multiple crystallites form a grain boundary or the inter-crystallite interface. A grain boundary is a structural defect that hinders the efficient directional transfer of mechanical stress or thermal phonons in crystal aggregates. We observed that grain boundaries within an aggregate of a-few-nanometers-wide fibrillar crystallites of wood cellulose were crystallized by enhancing their inter-crystallite interactions; multiple crystallites were coupled into single fusion crystals without passing through a melting or dissolving state. Accordingly, the crystallinity of wood cellulose, which has been considered irreversible once decreased, was significantly enhanced, and the thermal energy transfer in the aggregate was improved. Other fibrillar crystallites of crab shell chitin also showed a similar fusion phenomenon by enhancing the inter-crystallite interactions. These findings imply that such crystallite fusion naturally occurs in biological structures with network skeletons of aggregated fibrillar crystallites.