High tensile strength fibers of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] [P(3HB-co-3HH)], a type of microbial polyesters, were processed by one-step and two-step cold-drawn method with intermediate annealing. Thermal degradation behaviors were characterized by differential scanning calorimeter and gel permeation chromatography measurements. Thermal analyses were revealed that molecular weights decreased drastically within melting time at a few minute. One-step cold-drawn fiber with drawing ratio of 10 showed tensile strength of 281 MPa, while tensile strength of as-spun fiber was 78 MPa. When two-step drawing was applied for P(3HB-co-3HH) fibers, the tensile strength was led to 420 MPa. Furthermore, the optimization of intermediate annealing condition leads to enhance the tensile strength at 552 MPa of P(3HB-co-3HH) fiber. Wide-angel X-ray diffraction measurements of these fibers suggest that the fibers with high tensile strength include much amount of the planer-zigzag conformation (β-form) as molecular conformation together with 21 helix conformation (α-form). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41258.