Low-molecular polyethylene (LMPE) as an internal flexibilizer was selected to improve the spinnability of cellulose solution in N-methylmorpholine-N-oxide (NMMO). It is compatible with the conditions of cellulose dissolution and fiber spinning because of its chemical stability and melting point. In the course of rheological investigations, it was proven that this modification has a physical characteristic. Cross sections of the fibers reveal that cellulose-LMPE forms a two-phase system with randomly distributed LMPE domains and a dominant skin-core structure. The addition of 5% LMPE is enough to improve significantly the mechanical properties of the fibers. An increase of LMPE above 5% worsens these properties, because LMPE is not a fiber-forming polymer. With the use of spinnerets with suitable dimensions, it is possible to obtain cellulose fibers with mechanical properties similar to those of synthetic fibers. The retention of water is lowered by the hydrophobic polyethylene polymer; however, it is high enough to retain the hydrophilic character of cellulose fibers.