Abstract: A method which is a combination of meso-theoretical analysis and macro-experimental measurement was first given to calculate the in situ fiber modulus by using the mixture law and the relationships between the stress-strain hysteresis characteristics and the constituent modulus. The method was successfully applied to C/SiC composites. The result from preliminary experiments shows that the method is effective. It also shows that the in situ fiber modulus degrades noteworthily in C_f/SiC composites.