Investigation of the factors affecting the carbonization process is very important for the manufacture of desired, on-demand carbon fibrous morphologies. In this work, the effect of the average fiber diameter on the carbonization of precursor polymer fibers into carbon fibers was examined. Three electrospun fibrous mats consisting of a lignin/recycled PET blend with mass ratio of 1/1 and having different average fiber diameter (80, 387 and 781 nm) were prepared. After they were carbonized at 600 degrees C, it was found that the thicker fibers (387 and 781 nm) yield well-formed carbon fibrous morphologies, with average diameter of the same range as the precursor ones. In contrast, the thinnest nanofibers with an average diameter of 80 nm fuse with each other and lose their fibrous morphology, due to the maximization of heat and mass transfer during the process. These results highlight the decisive role of the nanoscale dimension in processes controlled by heat and mass transfer phenomena, as in the case of carbon fiber manufacture.