The isothermal crystallization and crystal morphology of poly(trimethylene terephthalate) (PTT)/poly (ethylene 2,6-naphthalate) (PEN) blends were investigated with differential scanning calorimetry and polarized optical microscopy. The commonly used Avrami equation was used to fit the primary stage of isothermal crystallization. The Avrami exponents were evaluated to be in the range of 3.0-3.3 for isothermal crystallization. The subsequent melting endotherms of the blends after isothermal crystallization showed multiple melting peaks. The crystallization activation energies of the blends with 20 or 40% PTT was -48.3 and -60.9 kJ/mol, respectively, as calculated by the Arrhenius formula for the isothermal-crystallization processes. The Hoffman-Lauritzen theory was also employed to fit the process of isothermal crystallization, and the kinetic parameters of the blends with 20 or 40% PTT were determined to be 1.5 x 10(5) and 1.8 x 10(5) K-2, respectively. The spherulite morphology of the six binary blends formed at 190 degrees C showed different sizes and perfect Maltese crosses when the PTT or PEN component was varied, suggesting that the greater the PTT content was, the larger or more perfect the crystallites were that formed in the binary blends. (c) 2006 Wiley Periodicals, Inc.