Whey protein isolate – WPI/cellulose fibers were obtained combining formation of cellulose fibers with acid induced gelation of whey protein in the simultaneous process. The mechanical properties of the cellulose fibers were improved by adding whey protein. Regenerated cellulose fibrils exhibited a dense structure with voids of several micrometers pore size. The blend fibers were smoother than cellulose fibers with smaller void spaces and attached protein filaments. On the surface of the regenerated cellulose fibers, formation of hierarchical flowerlike micro/nanostructures was observed. The XRD pattern indicated that the flowerlike microstructure was composed of ammonium copper(II) sulfate hexahydrate. Regenerated cellulose alone and in the mixtures with WPI crystallized partly in the cellulose II monoclinic system. For cellulose/WPI blend reduction in –C–OH groups was observed, which was probably caused by interactions of cellulose with the protein molecules. The obtained blended cellulose/WPI fibers can have tremendous potential use in medicine, tissue engineering and as a material for active ingredients release.