Ultimate mechanical properties of polyethylene fibers were measured. Results are in close agreement with the stress-induced melting theory of fracture (for finite molecular weight polymers). The perfect fiber work of rupture W-c, modulus K-c, strength sigma(c), and strain epsilon(c) are found to be W-c = 0.084 +/- 0.003 GPa; sigma(c) = 7.5 GPa; K-c = 335 +/- 12 GPa; epsilon(c) = 0.0225 +/- 0.0005. The activation energy of fracture is measured as approximate to 108 kJ/mol-the activation energy of polyethylene fusion and one-third the activation energy of bond scission. Nonuniformity of fibers necessitates averaging properties over several test lengths. Actual stress-strain curves are decomposed into thermodynamic and irreversible components. Fusion theory applies to the thermodynamic component.