The differential laser interferometry method and its application for measuring the thermal expansion of samples is described. The thermal expansion of single-crystal Al2O3 (corundum) was measured up to 1000 K with the differential laser-interferometer. The changes in the distance between two faces of a stepped shape sample are measured. The main feature of the apparatus is simultaneously monitoring two fringe signals that are 90° out of phase. This improvement of the basic laser-interferometer allows us to remotely measure the thermal expansion of minerals with high precision. The overall sensitivity with which we can detect changes in length is about 1/100 of the wavelength of the He-Ne laser (1/100 of 0.6328 × 10−6 m). After adjusting for errors caused by heat, the thermal expansion data measured with the differential laser-interferometer are in good agreement with previous reports. Use of this apparatus allows the investigation of the high-temperature thermodynamic properties of important rock-forming minerals.