Nylon 66 microfibers were obtained by a carbon dioxide (CO2) laser-thinning method. A laser-thinning apparatus used to continuously prepare microfibers consisted of spools supplying and winding the fibers, a continuous-wave CO2-laser emitter, a system supplying the fibers, and a traverse. The diameter of the microfibers decreased as the winding speed increased, and the birefringence increased as the winding speed increased. When microfibers, obtained through the laser irradiation (at a power density of 8.0 W cm(-2)) of the original fiber supplied at 0.23 m min(-1), were wound at 2000 m min(-1), they had a diameter of 2.8 mu m and a birefringence of 46 x 10(-3). The draw ratio calculated from the supplying and winding speeds was 8696x. Scanning electron microscopy showed that the microfibers obtained with the laser-thinning apparatus had smooth surfaces not roughened by laser ablation that were uniform in diameter. To study the conformational transition with winding speed, the changes in trans band at 936 cm(-1) andgaucheband at 1136 cm(-1) were measured with a Fourier transform infrared microscope. The trans band increased as the winding speed increased, and the gauche band decreased. Young's modulus and tensile strength increased with increasing winding speed. The microfiber, which was obtained at a winding speed of 2000 m min(-1), had a Young's modulus of 2.5 GPa and tensile strength of 0.6 GPa. (c) 2005 Wiley Periodicals, Inc.