Immiscible blends of cellulose acetate butyrate (CAB) and isotactic polypropylenes (iPPs) with different melting index were extruded through a two-strand rod die. The extrudates were hot-drawn at the die exit at different draw ratios by controlling the drawing speed. The morphologies of iPP fibers extracted from the as-obtained extrudates after removal of CAB by acetone were investigated by scanning electron microscopy. The influences of draw ratio, viscosity ratio, and composition ratio of CAB/iPP on the morphology evolution of iPP phase into nanofibers in the immiscible blends were studied. It was found that the thermoplastic iPP nanofibers were formed from the elongation of iPP ellipsoids, end-to-end merging of elongated iPP microfibers, and the size decrease of iPP microfibers in the processes of extrusion and drawing. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 921-931, 2010