Effect of the spinning speed and the heat treatment on the crystallization behavior of a PEN (polyethylene 2,6-naphthalene dicarboxylate) fiber was investigated by analyzing the experimental data on density, birefringence, thermal stress, DSC thermogram and wide angle X-ray diffractogram. Also a study on the differences in the crystallization behavior between PEN and PET (polyethylene terephthalate) fibers was carried out. The experimental results show that melting temperature of PEN fiber increases stepwise with the increase in the spinning speed. The half time of crystallization reaches the lowest value at 20$0^{circ}C$, which means the highest Crystallization rate. An abrupt increase in the degree of crystallization and birefringence was also found at the spinning speed of 3,500 m/min. The PEN fiber contains a $eta$ crystal at the spinning speed of 5,500 m/min, which was converted to an $alpha$ one by the fixed length heat treatment. The $eta$ crystal form, however, was not converted by heat treatment at the tension of 0.0088 N/tex. The heat treatment under tension of as-spun fibers, wound at the speed of 4,500 and 5,500 m/min yields more $alpha$ crystals at 22$0^{circ}C$ than at 17$0^{circ}C$.