Solid state transformation of crude poly (ethylene terephthalate)(PET) and PET/nano-CaCO3 (MPET) composites were studied by variable temperature FTIR spectroscopy during the heating process from 40 to 250 ℃. The effects of nanometer calcium carbonate(nano-CaCO3) on the solid state transformation and crystal correlation bands of MPET composites were analyzed by the curves of the ratio of 1 342 and 1 410 cm-1 absorbency(A1 342/A1 410) with temperature increasing, and together with DSC curves in the same condition. The results showed that the crystallization degrees of crude PET and MPET are obviously different in this condition by adding nano-CaCO3 particles as inhomogeneous nucleating agents.