Vapor-grown carbon fibers (VGCF) are a type of short carbon fibers, not yet commercially available, but with a potentially excellent cost/performance ratio owing to the simplicity of their production process. Scanning electron and atomic force microscopy studies have been performed. In each study, a similar structure was observed at the micrometer scale, the so-called "tree-trunk" structure, consisting of concentric cylindrical layers. This atomic force microscopy study revealed the nanometer structure. The layers are composed of several nanofibers approximately 200 nm in width, each exhibiting a fine structure of nanofibers approximately 15 nm in width. This nanostructure is similar to the ex-PAN fibers and explains the acceptable level of mechanical properties obtained for VGCF.