Abstract: The thermal expansion coefficients of fiber composites are studied in this paper. A method of measuring the thermal expansion coefficient by piezoelectric photoacoustic technique was proposed. An intensity-modulated Ar⁺ laser beam was used as the incident light. The beam was focused on an about 1μm diameter spot and illuminated collinearly the sample surface. Using the technique, piezoelectric photoacoustic signals as a function of different frequencies were experimentally measured. The thermal expansion coefficients can be obtained by fitting the experimental data. On the other hand, the thermal expansion coefficients of one-way composite C/C and C/Al in the transverse, longitudinal directions were measured. The measured results are reliable by comparing with other calculation methods and experimental results. Thermal expansion coefficients in arbitrary directions of C/C and C/A were given. The defects of the theoretical calculation method, which is complicated and by which the thermal expansion coefficient in arbitrary directions can not be measured, are overcome.