Thermoplastic nanofibers in yarn form were prepared by melt extrusion of immiscible blends of cellulose acetate butyrate (CAB) and thermoplastic polymers and subsequent removal of the CAB matrix. Isotactic polypropylene (iPP) nanofiber, with diameters ranging from 75 to 375 nm, were made by using CAB/iPP blends, with a ratio of 97.5-2.5, and a hot-drawn ratio of 25 in a twin-screw extruder. Dispersion of iPP in CAB and shear and elongational deformation are major factors to result in nanosized fibers. The spheres and nanofibers with different diameters of iPP dispersed phases in the CAB matrix can be well controlled by changing the flow field and the blend ratio. Differential scanning calorimeter and wide angle X-ray diffraction results showed that the nanofibers had lower crystallinity and crystallite thickness compared with bulk iPP. The atomic force microscopy images presented the well-defined nanofiber morphology and the excellent manipulability of single iPP nanofiber separated from a bundle of iPP nanofibers.