Development of high throughput production processes for making thermoplastic nanofiber and nanofiber yarns are urgently needed. PET, PTT, and PBT nanofibers were prepared from PET/CAB, PTT/CAB, PBT/CAB immiscible polymer blends by in situ microfibrillar formation during the melt extruding process. The diameter distribution and crystallization properties of PET, PTT, and PBT nanofibers were analyzed. After removing the CAB matrix phase, the nanofibers could be collected in the forms of random or aligned nanofibers and nanofiber bundles or yarns. To understand the formation mechanism of the nanofibers, the morphology development of three different polyesters in the dispersed phase were studied with samples collected at different zones in a twin-screw extruder. The morphological development mechanism of the dispersed phases involved the formation of sheets, holes and network structures, then the size reduction and formation of nanofibers. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012