Uniaxially oriented films with high tensile strength were processed from ultrahigh-molecular-weight poly[(R)-3-hydroxybutyrate] (P(3HB)) by a method combining hot-drawing near the melting point of the polymer and two-step-drawing at room temperature. In a two-step-drawn and subsequently annealed film, P(3HB) molecular chains fall into two states: 2/1 helix (α-forrn) and planar zigzag (β-form) conformations. The mechanism for generating the β-form during two-step-drawing was investigated by time-resolved synchrotron wide-and small-angle X-ray scattering measurements (WAXD and SAXS), together with the measurement of stress-strain curves. It was found that the improvernent of mechanical properties is due to not only the orientation of molecular chains but also the generation of the β-form during the drawing. The crystal and molecular structures of the α-form remained unchanged until the yield point of the stress-strain curve. At the yield point, the long period obtained from SAXS doubled and a new reflection indicative of the β-form was observed on the equatorial line in WAXD. The intensity of the reflection from the β-form increased with an increase in the two-step-drawing ratio at room temperature. The SAXS pattern changed from a two-point reflection along the meridian to a cross pattern with streaking on the equatorial line, demonstrating the close alignment of shish-kebab structures. The reflection intensity, crystal orientation and crystal size of the α-form decreased during two-step-drawing. Based on these results, the β-form is mainly introduced from the orientation of free molecular chains in the amorphous regions between α-form lamellar crystals, but the structural transformation of molecular chains also occurs from the α-form to the β-form at the deformed lamellar crystals.