Carbon fiber (CF) is recognized as an optimal reinforcement for the composites application areas which light weight and high mechanical properties are needed [1,5,11]. The use of CFs are increasing in the fields of military, aerospace, ship building, and sports goods due to these reasons. Additionally, the applications of CFs are becoming wider to the automobile industries. And it is even accelerating by the development of low cost and high performance CFs [1,2,5]. CFs can be classified by the origins of precursors. There are representatively polyacrylonitrile (PAN)-based, pitch-based and rayon-based fibers, but it is well recognized that PAN-based CFs have high mechanical properties [9]. Typical properties of PAN-based CFs are known as high tensile strength and modulus. These features are due to the ribbonlike microstructure in the fiber, and it is reported to cause the high performance of the fiber [3,8,10].