Numerous researchers have observed the bubble formation from C/C composite at high temperature (1300$^{circ}C$∼1700$^{circ}C$). According to thermodynamic calculation, the bubble can be formed at the temperatures above 1500$^{circ}C$ in the case of SiC coated C/C composite. However, the bubble below 1500$^{circ}C$ could not be explained. Therfore, in an effort to explain the low temperature bubble formation, the effects of inhibitors such as B, Al, Zr and Si were thermodynamically investigated along with hydrogen and water vapor impurities resolved in C/C composite and SiC coating layer.