In this study, mechanical and thermal properties of polyamide (PA) 6/66 with cellulose nano-fiber (CNF) treated by bisphenol fluorene diglycidil ether (BPFG), which is referred to as “B-CNF”, was investigated. Although in PA/neat CNF composite the CNFs were aggregated, B-CNF can be dispersed homogeneously in PA matrix and the size of B-CNF was getting smaller than that of CNF. On dynamic mechanical measurement, the storage modulus (E’) of PA/B-CNF was much higher than that of neat PA as well as PA/neat CNF, and the values of E’ increased with B-CNF content especially at high temperature region. On the other hand, the peak intensity of tanδ of PA/B-CNF at the temperature region around Tg of PA was getting lower in comparison with the neat PA and PA/neat CNF, indicating that the molecular motion of PA was restricted by the presence of B-CNF because of the strong interaction between PA and B-CNF. From the result of thermal mechanical measurement, Coefficient of Thermal Expansion (CTE) of PA/B-CNF was getting lower remarkably than those of neat PA and PA/neat CNF. Moreover, on the basis of CTE results, we estimated the volume fraction of PA’ phase, which interacted with B-CNF, by means of the “mixtures rule”, and as a result, it was found that the volume fraction of PA’ phase increased with the B-CNF contents and the values were much larger than those of PA/neat CNF. Furthermore, the volume fraction of PA phase, which was NOT interacted with CNF or B-CNF, had good correlation with the intensity of tanδ at Tg. These experimental results also supported our assumption mentioned above: the strong interaction existed between PA and B-CNF. Consequently, B-CNF was one of the most promising candidate to modify the mechanical properties of PA particularly at high temperature region.