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 a crystalline modification. The fibers processed at and above 3500 m/min were found to contain the b modification together with the a 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 a and b crystalline phases, indicating that not only the a but also the b 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.