High tensile strength melt-spun fibers of poly[(R)-3-hydroxybutyrate] (P(3HB)) with the addition of a small amount of ultra-high-molecular-weight P(3HB) (UHMW-P(3HB)) were processed by cold-drawing and two-step cold-drawing methods. The mechanical properties and highly ordered structure were investigated by tensile testing, and wide-angle and small-angle X-ray diffraction measurements (WAXD and SAXS) using a synchrotron radiation beam at SPring-8. The molecular weight of P(3HB) decreased drastically within a few minutes during melt-spinning. However, in the case of the blended samples, the degree of molecular weight decrease was slightly reduced by the addition of UHMW-P(3HB). The melt-spun blend fibers stretched at a cold-drawing ratio of 14 showed a tensile strength of 530 MPa. When two-step cold-drawing was applied, the tensile strength increased further to 740 MPa. WAXD and SAXD revealed that the addition of a small amount of UHMW-P(3HB) into P(3HB) induces the formation of the β-form (planar zigzag conformation) of molecular chain, increasing the mechanical properties. Based on these results, it seems that the addition of a small amount of UHMW-P(3HB) has a great effect on the thermal stability and processability of melt-spun fibers and increases their final mechanical properties.