In this study, we prepared several cellulose nano-fiber (CNF) by Cardo material (BPFG), referred to as B-CNF, with various degrees of substitution to BPFG. Moreover, we also prepared the CNF treated by Bisphenol-A diglycidyl ether (BPAG), which does not have Cardo structure in its chemical structure, and determined the effect of the difference in chemical structure on their mechanical properties at high temperature. TEM observation revealed that both of the CNF treated with BPFG (B-CNF) and that treated with BPAG (BisA-CNF) dispersed homogeneously in the PLA. The storage moduli of B-CNF/PLA composites above Tg increased with the degree of BPFG substitution. The storage moduli of B-CNF/PLA and BisA-CNF/PLA with similar degree of substitution were about 5 times higher than that of neat PLA above Tg, and the modulus of B-CNF/PLA was slightly higher than that of BisA-CNF/PLA. Moreover, the peak intensity of tanδ of B-CNF/PLA and BisA-CNF/PLA were lower than that of neat PLA, and the intensity of tanδ of B-CNF/PLA was lower than that of BisA-CNF/PLA. These results suggest that the molecular motion of PLA was restricted by the presence of B-CNF and BisA-CNF because the interaction between PLA and B-CNF or BisA-CNF is much stronger than that in untreated CNF/PLA. Furthermore, B-CNF improved the mechanical properties of the composite more effectively than BisA-CNF for the PLA composite above Tg. From the result of Coefficient of Thermal Expansion (CTE) measurements, we estimated the volume fraction of PLA phase interacting with B-CNF and BisA-CNF by means of the “mixtures rule”. As a result, it was found that the PLA phase interacting with of B-CNF/PLA is larger than that in BisA-CNF/PLA. This may be one of the factor which caused the difference in their mechanical properties of B-CNF/PLA and BisA-CNF/PLA. Finally, CTE measurements revealed that there is a strong relation between the intensity of tanδ at Tg and the volume fraction of the PLA interacting phase in PLA matrix.