X-ray ptychographic tomography has been recently developed for quantitative imaging of specimens on the nanometer scale. Here we present its application for the characterization of carbon fibers by mapping in three dimensions the mass density of entire fibers with diameters of several tens of micrometers with a resolution of about 100 nm. We characterized two fibers produced from two different precursors, revealing the spatial distribution of porosity and highly graphitized regions within the fibers. We further discuss the potential of ptychographic tomography as a new complementary technique for the characterization of carbon materials.