Poly(trimethylene terephthalate-block-tetramethylene oxide) (PTT-PTMO) copolymer/ graphene oxide nanocomposites were prepared by in situ polymerization. From the SEM and TEM images of PTT-PTMO/ GO nanocomposite, it can be seen that GO sheets are clearly well-dispersed in the PTT-PTMO matrix. TEM images also showed that graphene was well exfoliated into individual sheets, suggesting that in situ polymerization is a highly efficient method for preparing nanocomposites. The influence of GO on the two-phase structure, melt viscosity and mechanical properties of PTT-PTMO block copolymer was examined by using DSC, ARES rheometer and tensile tests. The DSC results imply that the introduction of GO did not affect the glass transition temperature of PTMO-rich soft phase, melting temperature of PTT hard phase and degree of crystallinity of the nanocomposites. As the graphene oxide loading in the nanocomposites increase, the enhanced Young’s modulus and yield stress was observed. The tensile strength slightly increased with the increase of GO from 0 to 0.5 wt% when elongation at break was higher or comparable to the value of neat PTT-PTMO copolymer.