The structural evolution in fibers produced by high-speed fiber spinning of Polyethylene 2,6 naphthalene dicarboxylate (polyethylene naphthalate) was investigated. The fibers were found to remain amorphous at speeds up to 2500 m/min, and subsequent increases in speed resulted in highly oriented crystalline domains containing primarily alpha crystalline modification. The fibers processed at and above 3500 m/min were found to contain the beta modification together with the alpha modification. At the highest speed investigated, 4000 m/min, the crystalline regions became disordered, and this was attributed to low deformation temperatures that accompany neck-like deformation. Constrained annealing of the fibers results in relatively unoriented crystalline structure at 500 m/min. Although the WAXS patterns of fibers spun in the 1000-2500 m/min range do not show any crystalline peaks, these fibers develop crystalline regions with significant orientation upon constrained annealing. In addition, these regions were found to have both alpha and beta crystalline phases, indicating that not only the alpha but also the beta phase can be grown from the oriented amorphous precursors upon constrained annealing. From our experimental results we extrapolated a value of 0.791 100% crystalline PEN. The estimated intrinsic birefringence value for the amorphous PEN is 0.75.