Display omitted The quantitative relationship between the microstructure behaviour and mechanical properties of polyacrylonitrile (PAN) carbon fibers (CF) after being heat treated in the range of 2000–2950 °C was studied. The surface morphology, elemental composition, orientation of graphite microcrystallites (Π), carbon interlayer spacing distances (d002), crystallite dimension (Lc and La), skin-core structure, pore structure and mechanical properties were characterised and evaluated after high-temperature heat treatment (HHT). The study found that increasing temperature resulted in an increase in the orientation of graphite crystals and the stress component of microcrystalline strong bonding direction along the fiber axial direction, leading to Young’s modulus of T300 and M40J increases of 18.7% and 7.7%, respectively. Meanwhile, the decrease in tensile strength by 53.9% and 29.4%, respectively, can be attributed to the deterioration of the homogeneity of the CF radial structure and a reduction in the stress dissipation path inside the CF. Finally, we have developed a theoretical model to predict the CF tensile strength and Young’s modulus based on Griffith’s fracture theory and uniform stress model, and it was verified with a relative error between the calculated and measured values less than 10%.