This study was performed to investigate the properties of mesophase pitch based carbon fibers during stabilization. The stabilization of those was carried out in the range of 230, 280, 320℃ under atmospheric condition for 0-3 hours. The characteristics of the reactions were observed by chemical and optical analysis. The condition of stabilization was determined by testing the mechanical properties of carbonized fibers. The stabilization of mesophase pitch based carbon fibers was progressed with permeating oxygens to fibers, escaping hydrogens and carbons from those. The skin-core structure, a random and mosaic structure is propgressed by oxidant from the surface of stabilized fibers to the center of those, was confirmed by using the polarized micrograph. The cross-sectional structure of carbonized fibers was controlled by the structure of stabilized fibers. The mechanical properties of carbonized fibers were determined using the stabilization time tempera-ture and the introduced oxygen contents. The tensile strength and Young’s modulus of carbonized fibers stabilized at 320℃ for 1 hour with 8.9wt% of oxygen contents were founded to be superier to others.