Blends of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) with various compositions were prepared using a mixer at 290 degrees C and 100 rpm for various times. The transesterification level f(TEN) and the average chain lengths L-PET and L-PEN were determined using H-1 NMR. The dynamic mechanical properties of PET/PEN blends were studied as functions of blend composition and transesterification level. This technique was found to be sufficiently sensitive to observe the change in the physical state and the molecular motions of PET/PEN blends in the glass transition and the subsequent cold-crystallization regions. For the PET/PEN blends having f(TEN) of above 10%, all the blends quenched from molten state exhibited the same dynamic mechanical behavior in the glass transition region showing a single glass transition temperature T-g. T-g of the blend was a linear function of the blend composition and independent of f(TEN). For the PET/PEN blends having f(TEN) of about 5%, PET/PEN = 75/25 and 25/75 blends formed a one-phase system, while 40/60, 50/50, and 60/40 blends formed a two-phase system. On the other hand, the cold-crystallization behavior detected by abrupt increase in storage modulus E＇ was observed in the temperature range from T-g to T-m (melting temperature) and dramatically influenced by the average chain length. Cold crystallization was observed for the samples having L-PET or L-PEN larger than 5, while lost for both L-PET and L-PEN less than or equal to 5. The effect of, blend composition on miscibility has also been discussed.