An isotactic polypropylene (i-PP) microfiber was continuously produced by using a carbon dioxide (CO2) laser-thinning apparatus developed in our laboratory. The CO2 laser-thinning apparatus could wind up the obtained microfiber in the range of 100 m min(-1) to 2500 m min(-1). The diameter of the microfiber decreased and its birefringence increased with increasing winding speed. When the microfiber obtained by irradiating the CO2 laser operated at a power density of 31.8 W cm(-2) to the original fiber supplied at 0.30 m min(-1) was wound at 1,387 m min(-1), the obtained microfiber had a diameter of 3.5 mu m and a birefringence of 25 x 10(-3). The draw ratio calculated from the supplying and the winding speeds was 4,623-fold. The SEM photographs showed that the obtained microfibers had a smooth surface without a surface roughened by a laser-ablation and were uniform in diameter. The wide-angle X-ray diffraction photographs of the microfibers wound at 848 and 1,387 m min(-1) showed the existence of the oriented crystallites. (c) 2005 Wiley Periodicals, Inc.