PAN-based carbon fibers were subjected to high temperature heat treatment (HTT) at 1500–2500°C and the evolution of the skin-core structure of the carbon fibers was investigated by elemental analysis, XRD, TEM and Raman spectroscopy. Results showed that nitrogen content and d 002 decreased, and carbon content and La increased with HTT temperature. La increased sharply with temperature above 1900°C. The Raman spectra taken at four points in radial directions indicated that the intensity of the G band at the skin was stronger than at the core with no change in peak position at 1700°C. However, the G peak position decreased from 1588.2 to 1582.2cm −1 , the width of the G band at half maximum intensity decreased from 46.3 to 28.9 and that of D band from 46.9 to 36.7cm −1 from core to skin after HTT at 2500°C. The differences of microstructure in the core and skin at different temperatures are related closely to the differences in nitrogen removal efficiency and temperature. At temperatures below 1900°C, nitrogen near the skin is more easily removed due to a shorter diffusion path and a higher temperature on the skin, leading to a slower increase of La with temperature on the skin than in the core. At temperatures above 1900°C, nitrogen is removed completely and graphitization on the skin is more rapid than in the core, leading to a large skin-core difference with the skin portion having a high modulus and the core portion a high strength.