A small carbon fiber diameter is one of the most important objectives in carbon fiber technology. This produces improved mechanical properties and a larger surface area per unit mass. Electro-spinning is able to produce polymer fibers with diameters ranging over several orders of magnitude, from the micrometer to the nanometer range. Optimization of electro-spinning process to produce the smallest nanofibers from high molecular weight emulsion polymerized PAN polymer with dimethylformamide has been investigated. Relationships between Berry’s number, spinning height, spinning angle and charge density and the diameter of the electro-spun PAN nanofiber have been established.