In preliminary experiments to optimize the condition of a laser heating, zone drawing for poly(ethylene terephthalate) (PET) fiber, a microfiber was prepared by a continuous-wave carbon dioxide (CW CO2) laser heating. CW CO2 laser heating was carried out at an extremely low applied tension (sigma(a)) at a higher laser power density (PD) as compared to the optimum condition for the laser heating, zone drawing of PET fiber reported previously. The microfibers were obtained by CO2 laser heating carried out at a PD of 15.8 W cm(-2) and under a sigma(a) of 0.66 MPa or lower. The diameter of the fiber decreased with a decreasing sigma(a) and increasing PD. The smaller the diameter, the higher was its birefringence. The smallest diameter fiber obtained at sigma(a) = 0.17 MPa at PD=21 W cm(-2) had a diameter of 4.5 mum and a birefringence of 0.112, and its draw ratio estimated from the diameter reached 3086 fold. Such a high draw ratio was not previously attained by any drawing method. In a wide-angle X-ray diffraction photograph of the smallest diameter fiber, indistinct reflections due to oriented crystallites were observed. An SEM micrograph of the smallest diameter fiber showed a smooth surface without any crack and was uniform in diameter. (C) 2003 Wiley Periodicals, Inc.