Mechanical and thermal properties of poly(lactic acid) (PLA) with cellulose nanofiber (CNF) treated by Bisphenol fluorene diglycidyl ether (BPFG), B-CNF, was investigated. When using neat CNF to obtain PLA/CNF composite, the CNFs were aggregated together and the average dispersed size was about 1 m. When using B-CNF to obtain PLA/B-CNF composite, however, B-CNF can be dispersed homogeneously in PLA matrix and the size of B-CNF was getting smaller than that of CNF about several hundred nm. Furthermore, when employed master batch (MB) procedure, which is the method that PLA/B-CNF with high concentration of B-CNF (20 wt%) was prepared in advance, then it was blended with PLA to produce PLA/B-CNF (MB) composite at a given concentration of B-CNF, the CNFs can be dispersed much smaller and the size reached about 100 nm in PLA matrix. When comparing the storage modulus (E’) of PLA/CNF, PLA/B-CNF, and PLA/B-CNF (MB) at the plateau region above Tg, of PLA, the value of E’ of PLA/B-CNF (MB) was the largest, then PLA/B-CNF composite was the second, and PLA/CNF composite was the third, but the values of them were larger than that of neat PLA. On the other hand, when comparing the peak intensity of tanδ at the temperature region of Tg of PLA of these composites, it arranged in ascending in order: PLA/B-CNF (MB), PLA/B-CNF, PLA/CNF, and the largest one for neat PLA. These results suggested that B-CNF can play a role to restrict the molecular motion of PLA in comparison with neat CNF, especially B-CNF in PLA/B-CNF (MB) composite did much restrict the molecular motion of PLA. Besides, from the result of Coefficient of Thermal Expansion (CTE) of the composites, we estimated volume fraction of the interaction phase in PLA/CNF, PLA/B-CNF, and PLA/B-CNF (MB) by means of mixtures rule. As a result, it was found that the interaction phase in PLA matrix was the largest for PLA/B-CNF (MB), the second for PLA/B-CNF, and the third for PLA/CNF. There was a good relationship between the inten