A zone-drawing and zone-annealing method was applied to the poly(L-lactic acid) (PLLA) microfiber obtained by using a carbon dioxide (CO,) laser-thinning to develop its mechanical properties. The microfiber used for the zone-drawing and zone-annealing was prepared by winding at 800 in min(-1) the microfiber that was obtained by irradiating the laser at 8.0 W cm(-2) to an as-spun fiber supplied at a speed of 0.5 in min(-1) and had a diameter of 1.5 mu m and a birefringence of 20.3 X 10(-3). The zone-drawing was carried out at a drawing temperature of 85 degrees C under an applied tension of 150 MPa, and the zone-annealing at an annealing temperature of 110 degrees C under 181 MPa. The zone-drawing and the zone-annealing were carried out at a treating speed of 0.1 in min(-1). The diameter of microfiber decreased, and its birefringence increased stepwise with processing. The zone-annealed microfiber finally obtained had a diameter of 1.2 mu m, a birefringence of 43.3 X 10(-1), a tensile modulus of 14.0 GPa, and a tensile strength of 1.5 GPa. The wide-angle X-ray diffraction pattern of the zone-annealed microfiber showed the existence of the highly oriented crystallites. (c) 2006 Wiley Periodicals, Inc.