Isothermal crystallization behaviors of poly(trimethylene terephthalate) (PTT), poly(trimethylene naphthalate) (PTN), and PTT/PTN blends were analyzed using a polarizing microscope equipped with a hot-stage. As the melting time at $260^{circ}C$ of PTT/PTN blends increased, the crystallization proceeded slowly due to the copolymer formation which resulted from the ester interchange reaction at the elevated temperature. PTT and m-rich blends formed spherulites over the temperature range of experiment, but PTN formed spherulites only at lower crystallization temperatures such as $150^{circ}C$. PTN and PTN-rich blends formed distorted spherulites at high temperatures such as $180^{circ}C$, which seemed to result from the $eta-crystal$ formation of PTN at that temperature. The radial growth rate of the spherulite formed during the crystallization was obtained by the measurement of radii of the spherulites. The radial growth rate decreased as PTN content in the blends increased.