PEN fibers spun at 4 km/min by high speed melt-spinning were treated by two methods, constant length annealing (CLA) and high tension annealing (HTA) at 130, 160, 190, and $220^{circ}C$. The fine structure and physical properties of the PEN fibers were investigated with various techniques such as wide-angle X-ray diffraction, DSC thermo-grams, density, birefringence, tensile test etc. As the annealing temperatures is increased, a partial transition of the crystal structure, from $eta$ form crystal to $alpha$ form crystal appeared in the WAXD test in case of CLA treatment. But, in case of HTA treatment, the transition of the crystal form from $eta$ form crystal to $alpha$ form crystal appeared dominantly. As the annealing temperature and tension increases, the melting peaks become sharper in DSC thermograms. Density of PEN fibers from HTA treatment is lower than that from CLA treatment, but birefringence and initial modulus of the PEN fibers from HTA treatment are much higher than those from CLA treatment. But, shrinkage of the PEN fibers from HTA treatment is higher than that from CLA treatment.