Strain measurement of an unidirectionally solidified single-crystal Al2O3/single-crystal Y3Al5O12 (YAG) eutectic composite was performed by neutron diffraction technique using two-axis crystal diffractometer with one-dimensional position sensitive detector. Microstructure of this composite shows a formation of a compatible interface with no amorphous phase region among the two-phase grain boundaries. This fact causes excellent characteristics on high-temperature tensile and flexural strengths in the range of 350 similar to 400 MPa from the room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. The neutron diffraction measurements were carried out to measure residual strains of the two phases in the composite, together with the sintered specimens of AL(2)O(3) and YAG as the reference materials, respectively. The YAG phase strains for three crystal lattice planes were successfully evaluated from the full set of the diffraction data But the corresponding strains for Al2O3, phase were not determined because the data set of Al2O3, phase was not unambiously obtained due to overlapping of the diffraction profiles from the two phases and/or no clear observation of the corresponding set of the peaks. It is shortly discussed that the overlapped profile results from the compatible interfaces with almost equal lattice spacings of the two phases. Contrary to the case of polycrystalline composites, the strains in this composite indicate a strong anisotropy because of the dual phase single crystal constituents.