A green composite was fabricated using bamboo fiber and 9,9′-bis(aryl)fluorene-modified cellulose nanofiber (FCNF). Cellulose nanofiber (CNF) and finely crushed bamboo fiber (CBF) were also used as binders. The mechanical properties of the composites were compared. It was found that the FCNF-bamboo fiber composite had the maximum flexural strength among these binders. This result was likely due to strong bonding by chemical reactions among fibers and the FCNF. The effect of fiber orientation accuracy on the mechanical properties of the composites was also investigated. When the bamboo fibers were carefully aligned, without fibers crossing each other, the mechanical properties increased by two times, compared to the composites with fibers crossing each other. In the accurately aligned bamboo composites, the cross sections of the fibers were largely deformed by compression stress during hot-press forming. Thus, the gaps among fibers decreased, and interfacial adherence was improved. The effect of fabrication temperature on the mechanical properties of the FCNF-bamboo composite was also examined. It was found that the maximum flexural modulus and strength of the composites were at approximately 250 °C, and the mechanical properties rapidly decreased above 270 °C due to thermal degradation of the bamboo fiber.