Polyethylene (PE) fibers were prepared by ethylene extrusion polymerization with an MCM-41-supported titanocene catalyst. The morphological and mechanical properties of these nascent PE fibers were investigated. Three levels of fibrous morphologies were identified in the fiber samples through an extensive scanning electron microscopy study. Extended-chain PE nanofibrils with diameters of about 60 nm were the major morphological units present in the fiber structure. The nanofibrils were parallel-packed into individual inicrofibers with diameters of about 1-30 mum. The microfibers were further aggregated irregularly into fiber aggregates and bundles. In comparison with commercial PE fibers and data reported in the literature, the individual microfibers produced in situ via ethylene extrusion polymerization without post-treatment exhibited a high tensile strength (0.3-1.0 GPa), a low tensile modulus (3.0-7.0 GPa), and a high elongation at break (8.5-20%) at 35degreesC. The defects in the alignment of the nanofibrils were believed to be the major reason for the low modulus values. It was also found that a slight tensile drawing could increase the microfiber strength and modulus.