The effects of spinning conditions on the fibrillation process of polymer blends, on the example of poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVA) blend systems, in an elongational flow within fiber formation zone are systematically and thoroughly investigated for the first time. By considering the relationship between the changes in filament parameters with the focus on the maximum axial strain rate (ASR) and tensile stress at maximum ASR and the morphological evolution of the dispersed PLA phase along the spinline, the fibrillation process from rod-like to nanofibrillar structures of the dispersed PLA phase in a binary blend with PVA matrix is elucidated. The final morphology of the dispersed PLA phase in PLA/PVA blends is controlled by the changes in the spinning conditions. The lengths and diameters of the PLA fibrils are caused not only by only the deformation of their initial sizes but by the combination of the deformation, coalescence, and break-up process.