Blend films containing wild-type poly[(R)-3-hydroxybutyrate]] (P(3HB)) and ultrahigh-molecular-weight P(3HB) (UHMW-P(3HB)) (compositions of 5/95 and 10/90 w/w) were prepared by solvent-casting and subsequent cold-drawing. The thermal properties, crystallization behavior, mechanical properties, and highly ordered structure of the blend films were analyzed by differential scanning calorimetry, polarized optical microscopy, a tensile test, and wide- and small-angle X-ray diffraction measurements with synchrotron radiation. The maximum radial growth rate of spherulites and corresponding temperature were identical for films of different composition. However, the half-time of crystallization of the blend films was shorter than that of P(3HB) because UHMW-P(3HB) behaves as a nucleating agent. The tensile strength, Young's modulus, and elongation at break of a 5/95 blend film after cold-drawing to 12 times of the original length were 242 MPa, 1.50 GPa, and 88%, respectively, which are higher than those of an UHMW-P(3HB) cold-drawn film and similar to those of common plastic films as poly(ethylene terephthalate). The wide-angle X-ray diffraction results indicated that the cold-drawn films with high tensile strength contained both 2 1 helix (α-form) and planar zigzag (β-form) conformations. Addition of a small amount of UHMW-P(3HB) to P(3HB) caused the β-form to appear in blend films at a high drawing ratio. A mechanism for forming β-form crystals in the blend films is proposed. Enzymatic degradation of the cold-drawn blend films is demonstrated using polyhydroxybutyrate depolymerase, suggesting that the rate of enzymatic degradation can be controlled by addition of UHMW-P(3HB).