A network of nanofibers is formed in situ through solid-state deformation of disentangled ultra-high molecular weight polyethylene (dis-UHMWPE) during compounding with a polyolefin elastomer below the melting temperature of dis-UHMWPE crystals. Dis-UHMWPE was prepared in the form of powder particles larger than 50 μm by polymerization at low temperatures, which favored the crystallization and prevention of macromolecules from entangling. Shearing the blend for different durations and at different temperatures affects the extent to which the grains of dis-UHMWPE powder deform into nanofibers. Disentangled powder particles could deform into a network of nanofibers with diameters between 110 and 340 nm. The nanocomposite can be further sheared for a longer time to decrease the diameter of dis-UHMWPE nanofibers below 40 nm, being still composed of ≈70 wt.% of crystalline and ≈30 wt.% of amorphous components. Subsequently, these thinner fibers begin to melt and fragment because they are thinner and also because the amorphous defects locally decrease the nanofibers’ melting temperature, which results in their fragmentation and partial loss of nanofibers. These phenomena limit the thickness of dis-UHMWPE nanofibers, and this explains why prolonged or more intense shearing does not lead to thinner nanofibers of dis-UHMWPE when compounded in a polymeric matrix.