Thermal expansion is determined by measuring interferometrically the relative displacement of two fibers attached to specimens. Coherent monochromatic light from a laser incident upon the two adjacent fibers produces a Fraunhofer fringe pattern. Relative displacement of the fibers can then be determined by monitoring the motion of the fringe pattern. The technique was applied to the measurement of thermal expansion of ceramics in air to 1150 °C and of graphites in vacuum to 1900 °C.