The volume fractions of constituent phases are a rudimentary parameter for ceramic composites. Synchrotron-radiation computed tomography (SR-CT) is an emerging three-dimensional characterization method, and this article is intended to explore its application in determination of the phase volume fraction for ceramic composite. The composite of yttria-stabilized zirconia (YSZ) and Sr-doped LaFeO3 (LSF) was used as a model material to examine the validity of the method. A three-dimensional image of the composite was obtained using the SR-CT method. The constituent phases of the composite were discriminated by the gray level of the pixels in the image, and their volume fractions were determined by counting the number of corresponding pixels. The volume fractions for YSZ and LSF were 56.8% and 36.6%, respectively, and the volume ratio of these two phases was 1.55, slightly higher than that calculated from the composition of the starting powders. The porosity of the composite determined by SR-CT was 6.6%, somewhat lower than that by the Archimedes method. For comparison, the phase volume fractions of the composite were also determined by analyzing its backscattered electron-SEM images, and the results of this two-dimensional method are in fair agreement with those of SR-CT. It is concluded that SR-CT with improved spatial and compositional resolution can give a fairly accurate measurement of the phase volume fraction of the ceramic composites.