Carbon fiber reinforced carbon (C/C) composites have light weight, high strength, high elastic modulus and high temperature strength in an inert atmosphere. However, carbon materials can actively react with oxygen in oxidation environment at above 500°C. Therefore, it is necessary to investigate about the oxidation mechanism of C/C composites on basis of grahitized microstructure. And also, oxidation behaviors of C/C composite made by different types of carbon fiber have not been made clear. In this study, oxidation behavior of C/C composites with different two types reinforcing fiber were investigated at high temperature in air by measuring weight loss with oxidation time and by examining it's graphitization and microstructure. Then, mechanical properties of oxidized C/C composites were investigated by 3-point bending test under span ratio of 4. As a result, the oxidation rate of C/C composites with Carbon Fiber heat-treated at higher temperature shows smaller value than that of Carbon Fiber heat-treated at the lower temperature. Furthermore, the difference in the oxidized morphologies is recognized dependending upon the oxidation temperature and the type of Carbon Fiber. And, in mechanical properties, the reduction of shear strength was preferentially recognized in case of the specimen oxidized at 600°C comparing to that oxidized 1000°C, even when the total weight loss is almost the same.