The structure of a mesophase pitch carbon fiber is largely developed during the fiber spinning process and is perfected in stabilization and carbonization steps. It is of critical importance to strictly control the extrusion parameters to form standard carbon fibers. Designs of experiments were employed in melt spinning under 17 conditions to investigate the influence of three process variables - screw velocity, spinning temperature and winding speed - on the diameter of petroleum-derived pitch filaments. Screw velocity was found to be the most significant factor to produce thinner fibers. Optimized conditions were validated in an experiment that produced a filament with average diameter of 14 micrometers, proving that design of experiments is a powerful tool to be used in carbon fibers production. The data collected on experiments were used to model the elongational flow that occurs during the fiber formation. Tensile strength at which the material can be subjected under extrusion without breaking was evaluated. The relation between the degree of orientation of the carbon layers parallel to the fiber axis and the diameter of the fibers was also analyzed. Despite the fact that the thickness of the fiber improved the axial alignment of molecules, there is a limiting diameter at which the diffusion of oxygen, needed to form stabilized fibers, do not occur. Only diameters smaller than 45 micrometers could be fully stabilised in air. Insufficient stabilization of the thicker filaments induced defects on their structure that reflected on their tensile strength and modulus. While thinner fibers reached strengths of 1000 MPa and 90 GPa modulus, thicker fibers have only supported 200 MPa with 17 GPa modulus.