Effect of twice step heat treatment on mechanical properties and microstructure of graphite fibers was
studied,and the skincore degree of graphite fibers was analysed by Raman spectroscopy. The results found that twicestep heat treatment contributed to increasing the tensile strength and volume density of graphite fibers,but showed lesseffect on tensile modulus of graphite fibers. From the results of Raman spectroscopy,we found that the surface graphitization degree was lower,the inner graphitization degree was higher and the skincore gene was larger by twice step heattreatment,which meant that graphite fiber was more homogeneous. Twice step heat treatment was good for inreasingcompactness and homogeneousness of graphitize fibers,which was the reason for tensile strength increasing