In our study, we present atomic force microscopy (AFM) investigations of the surface of Polyacrylonitrile-based carbon fibers utilizing two different AFM probes, a standard tip as used in literature up to now and a recently made available super sharp tip. Using the super sharp tip, we identified so far not reported pore-like nanostructures distributed homogeneously over the surface of the fibers. We show that such nanopores are already present on the surface of the corresponding precursor fiber, indicating that these structures are characteristic for the fiber along the production process. To investigate a possible correlation between the surface structures and the mechanical properties of carbon fibers, we further analyzed the surface of various carbon fibers showing different tensile strengths. All investigated fibers show characteristic nanoporous surface structures and a correlation was found between the Nanopore size and shape and the mechanical properties. The effective nanopore area and the aspect ratio of the nanopores decrease with increasing tensile strength of the fibers. In addition, the nanoroughness of the fiber surface is correlated to the nanopore size and also decreases with increasing tensile strength.