Nylon 66 nanofibers were prepared by irradiating as-spun nylon 66 fibers with radiation from a carbon dioxide (CO2) laser while drawing them at supersonic velocities. A supersonic jet was generated by blowing air into a vacuum chamber through the fiber injection orifice. The fiber diameter depended on the drawing conditions used, such as laser power, chamber pressure, laser irradiation point, and fiber supply speed. A nanofiber obtained at a laser power of 20 W and a chamber pressure of 20 kPa had an average diameter of 0.337 μm and a draw ratio of 291,664, and the drawing speed in the CO2 laser supersonic drawing was 486 m s−1. The nanofibers showed two melting peaks at about 257 and 272°C. The lower melting peak is observed at the same temperature as that of the as-spun fiber, whereas the higher melting peak is about 15°C higher than the lower one. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 40015.