Poly(p-phenylene terephthalamide) (PPTA) nanofibers were prepared via a polymerization induced self-assembly process with the assistance of methoxy polyethylene glycol (mPEG) for stability and dispersity. In the traditional process of the solution polycondensation of p-phenylenediamine (PPD) and terephthaloyl chloride (TPC), PPTA will aggregate and precipitate with the chain growth due to the liquid crystalline characteristic. The introduction of mPEG can control the aggregation degree of PPTA molecules and stabilize the formed aggregates, which drives the self-assembly of PPTA molecules and results in the formation of nanofibers. The obtained nanofibers specialized by the great length–diameter ratio were characterized in detail. The effects of mPEG addition on the formation and the performance of the fibers were all studied in the work. The nanofibers can disperse in variety of organic solvents and water. Moreover, the nanofibers can be deposited for the formation of thin films with excellent transparency and thermal stability, which has great potential applications such as the separator of lithium ion battery.