Bacterial homo poly(3-hydroxybutyrate) (PI-IB) was melt spun into fibers and the relations among the processing conditions, crystalline structures, and the mechanical properties were investigated. WAXD patterns of PHB fibers indicate a broad reflection which cannot be assigned to any plane in the orthorhombic system of PHB, although most strong reflections can be assigned to the planes in the orthorhombic system. It has been reported that the broad reflection is assigned to be beta -zigzag form, which occurs by the stretching of the amorphous tie molecules between lamellae. When annealed under high tension, this broad reflection tended to be stronger, while it became significantly weak when annealed without tension. These results suggest that the annealing without tension makes the amorphous tie molecules crystallized into orthorhombic form, while annealing under high tension makes the molecules in the amorphous region stretched and increased the fraction of the beta -zigzag form. (C) 2001 Elsevier Science Ltd. All rights reserved.