The crystallization and morphology of poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) containing, as nucleating agents, a sodium salt of a copolymer of ethylene and acrylic acid or a sodium salt of a copolymer of ethylene and methacrylic acid, were investigated with differential scanning calorimetry, polarized optical microscopy, and small-angle light scattering. The nucleating agents accelerated the crystallization rate at high temperatures by decreasing the surface free energy barrier hindering nucleation. Meanwhile, the nucleating agents with flexible chains could also improve the mobility of the PEN chains and increase the crystallization rate at low temperatures. Hedrites were observed when PEN was crystallized at high temperatures, whereas crystallization at low temperatures led to the formation of spherulites. Similar but smaller morphologies were obtained in the presence of nucleating agents. With nucleating agents, the spherulites formed at low temperatures were less perfect, although the optical properties of the spherulites were not influenced.