In this work, the mechanism of fiber formation from polymer solutions under the action of gas jets in a process named gas jet fibers (GJF) method is studied. The GJF method relies on high velocity gas jet converting polymer solutions streaming from a nozzle into a liquid jet and then into fibers. Polymer nanofibers with diameter as low as 100 nm are continuously produced from polymer solutions. A series of interrelated phenomena including polymer jet initiation and stretching, flapping motion, bending instabilities, and concurrent solvent evaporation were found to influence fiber formation. Specifically, the behavior of the liquid jets, stretching rates, and the onset of the bending instabilities are revealed using high speed video photography. The results indicate that both the initial stretching from flapping and the extension from bending instabilities play important roles in fiber attenuation.