PAN/phenolic-based carbon/carbon composites are finding more and more use, and becoming more and more important for space/aviation industries, and general structural applications. The primary constituents of these materials are fragile, and cracking occurs through random failures caused by imperfectionsinduced random failures. The strength of a fragile material follows Weibull distribution. The variation of the flexural strengths has been modeled using Weibull distribution. In order to compute m and σu, first, the values of σi were ordered from the smallest to the largest, and then applying linear regression to these values. From the linear regression, the Weibull modulus and the characteristic strength were estimated. The CFRP composites show maximum flexural strength which decreases during the pyrolysis at the temperature of 1000 0C. The reduced flexural strength can be related to void defects. These defects probably act as a source of fractures during loading. The failure probabilities for CFRP and C/C composites are 0.55 and 0.78, respectively, and these values 334 and 92 MPa, and 29 and 10.2 GPa for mean flexural strength and flexural modulus, respectively, would be quite useful for understanding the fiber-matrix interfacial bonding properties, which have a strong influence on the mechanical properties of these composites.