3-hydroxybutyrate-co-4-hydroxybutyrate [P(3HB-4HB)] is one of polyhydroxyalkanoates which is biodegradable and biocompatible, and is one of the hot issues of the biodegradable materials research. But the degradation will occur at high temperature and shear rate. So P(3HB-4HB) is modified with PEG10000 by means of a mechanical blending. In this research, capillary rheometer and TGA are applied to investigate its rheological and thermal behaviors. The technology of melt spinning is also studied and the P(3HB-4HB)/PEG10000 fiber with a certain mechanical property is obtained successfully. Electronic velocimeter and electronic single end yarn strength tester are applied to investigate the mechanical property of the fiber. The result of TG indicates that the addition of PEG10000 improves the thermal stability of P(3HB-4HB). The result of capillary rheometer shows that P(3HB-4HB)/PEG10000 is pseudoplasticfluid. Because of the addition of PEG10000, the non-Newtonian index n of the P(3HB-4HB)/PEG10000 sample increases and the flow activation energy of the P(3HB-4HB)/PEG10000 sample decreases. The result of fiber strength test shows that the addition of PEG10000 has an impact on the mechanical property of the fiber and the lowest fiber strength appeared when PEG10000 content was P(3HB-4HB)/PEG=100/6. The moisture regain ratio increases with the content of PEG10000 rising. And when PEG10000 content is P(3HB-4HB)/PEG=100/2, the highest degree of orientation of fiber appears.