<p>The compressive strength of polyacrylonitrile-based high-strength medium-modulus carbon fibers (CFs) with different diameters was tested using monofilament rebound method. The crystalline structure, distribution of radial carbon structure and elastic modulus were studied by X-Ray Diffraction, Raman and Nanoindentation. The correlation between distribution of radial carbon structure and compressive strength was studied. The results showed that, under the premise of similar average crystallite size, the high-strength medium-modulus CFs with larger diameters exhibited significant differences in the skin–core structure, characterized by a larger microcrystalline size, a more perfect structure, and higher rigidity on the surface. Therefore, CFs with larger diameters had stronger resistance to compressive deformation, better resistance to compressive loads transmitted and dispersed from the core, resulting in higher compressive strength. When the diameter of high-strength medium-modulus CFs increased from 5.05 to 6.66&#xa0;μm, the compressive strength increased from 1.56 to 1.88 GPa, an increase of 20.51%. Their compressive failure modes were all buckling fracture. Therefore, increasing the diameter of CF within a certain range can not only improve the preparation efficiency of carbon fiber composites, but also effectively enhance the compressive strength, which provides the possibility for further weight reduction of composite structural components.</p> Graphical abstract <p></p>