To overcome the disadvantages of discontinuous conventional batch extruders, a continuous screw extruder is introduced to manufacture pitch-based carbon fibers. For a carbon fiber preparation process, an oxidation time of 8 h was determined to be optimal for obtaining desirable mechanical properties of the fibers acquiring employing the screw extruder. It is hypothesized that the differences in the properties of the carbon fibers fabricated utilizing the batch and screw extruders originate from the melt spinning time; therefore, a combined equation for the total amount of heat treatment from the pitch precursor through the oxidation process is established in this study. The crystallinity of the carbon fibers is confirmed to correspond to the differences in mechanical properties as the oxidation time increases. The poor mechanical properties of the carbon fibers that are insufficiently oxidized are a result of irregular oxidation from the sheath to the core of the fiber cross section. However, the over-oxidized carbon fibers also show poor mechanical properties than the optimal fibers. This result further affirms that excessive oxidation times cause unstable chemical bonding, which interrupts the formation of stable crystal structures after carbonization.