Poly(ethylene terephthalate)(PET)/poly(trimethylene terephthalate)(PTT) /Montemorillonite(MMT) nanocomposites were prepared by melt blending through a co-rotating twin screw extruder. The wide angle X-ray diffraction(WAXD) patterns proved the nanocomposites formed exfoliated or intercalated structures. The rheological properties of PET/PTT/MMT were studied by RH2000 capillary rheometer. The results show that PET/PTT and PET/PTT/MMT melts are all pseudo-plastic fluids, and the addition of one to five part MMT can obviously improve the flow property of PET/PTT melt. The apparent viscosity (ηa) of the nanocomposite melts is lower than that of PET/PTT melt, and their flow behavior is better than that of PET/PTT melt, so the nanocomposites can be processed at lower temperature. The viscous flow activation energy(ΔEη) of the nanocomposite melts is higher than that of PET/PTT melt, and it is more proper for the nanocomposite melts to improve their flow properties by increasing temperature.