Abstract: In vitro degradation of Polyglycolic acid（PGA）fiber by melting-spinning was investigated for a period of 9 weeks in phosphate buffered solution（pH=7.4）at 37 ℃.The degradation was studied with regard to the mass loss, tensile properties of the fiber, melting point, the degree of crystallinity and morphology of PGA fiber. The research showed that weight loss of PGA fiber during degradation was significant. After 2-week degradation, the weight loss speeded up obviously, and the weight loss reached about 80% at 6th week. But after 6-week degradation, the speed tended to be slow gradually. During the process of degradation, considerable changes in the mechanics and thermodynamics performance of PGA fiber had taken place. With duration of degradation, the melting peak moved to the low temperature and the melting point gradually reduced. The breaking force reduced gradually, and it basically had lost all of its strength after 4-week degradation. With duration of degradation, the diameters of PGA fibers became thinner; and crystallinity of PGA fiber in the degradation of 3 weeks gradually increased but gradually reduced afterwards.