Some unusual behavior in the temperature dependence of the Young’s modulus along the direction of the reinforcing graphite fibers in a graphite/aluminum composite has been observed. The decrease of the Young’s modulus with decreasing temperature as measured by ultrasonic techniques, utilizing noncontacting electromagnetic acoustic transducers, is shown to be due to the strong dependence of the fiber modulus on stress, which existed as a result of the mismatch between the coefficients of thermal expansion for the fiber and the matrix. Results appear significant in the selection of suitable combination of fibers and matrices for achieving thermally stable composites.