The development of carbon structures in mesophase pitch fibers was studied with controlling the initial temperature (Ti) of the stabilization process to optimize the mechanical properties of final carbon/graphite fibers. To understand the relationship between Ti and chemical structural change by thermostabilization, the pitch fiber was thermally treated from various Tis up to 350 °C with an elevation rate of 2 °C/min. Various analyses of the oxygen species in the fiber revealed that changing Ti governed the amount and distribution of oxygen in the stabilized fibers, while determining the effective duration of thermostabilization and preserving hydroxyl groups originally present in the precursor fiber. Among various samples, stabilized fibers with Ti of 150 °C contained the largest amount of oxygen in the fiber, resulting in the highest degree of polyaromatic inner structure after the subsequent carbonization and graphitization to show the best tensile strength and modulus. We also demonstrated that the optimal mechanical, electrical, and thermal properties of the resulting graphite fiber were comparable to those of commercially available fiber products (XN-80-60s), indicating that Ti should be carefully considered to enhance the properties of carbon and graphite fibers.