Cellulose nanofiber (CNF) modified with fluorene derivative (BPFG ： Bisphenol fluorene diglycidyl ether), which has cardo moieties (FLCF ： Fluorene modified cellulose nanofiber), was successfully synthesized, and we evaluated the properties of composites of polyamide 12 (PA 12) by X-ray CT, DMA, tensile test, DSC and FE-SEM. As a result, it was revealed that FLCF was uniformly dispersed in PA 12, while some aggregates were formed in the case of unmodified CNF. Furthermore, FLCF had a high reinforcing effect for PA 12 including storage modulus and tensile strength even in a temperature range above the glass transition temperature compared with unmodified CNF. These effects are attributed to the interaction between FLCF and PA 12 being improved by enhancing the affinity derived from bulky structure and hydrophobic nature of BPFG, thereby improving the physical properties. This result suggests that modifying CNF by fluorene derivatives such as BPFG is a promising approach for adapting CNF to resin reinforcement applications.