Coarsening of the fine lamellar structure of a directionally solidified Y3Al5O12 (yttrium aluminum garnet, YAG)/Al2O3 eutectic fiber at elevated temperatures was investigated. The fibers were grown continuously by an edge-defined film-fed growth (EFG) technique. To study the thermal stability of the lamellar structure, the fibers were heat-treated in air at 1360 degrees -1460 degreesC for up to 200 h. X-ray diffractometry and scanning electron microscopy were used to characterized the microstructures of the fibers. Image analysis was used to measure the length of the interface line between Y3Al5O12 and Al2O3 phases. The kinetics of coarsening and the rate-controlling mechanisms were investigated. Also, the Graham and Kraft model for describing the coarsening behavior of the lamellar Al-CuAl2 eutectic alloy was used to explain the coarsening behavior of Y3Al5O12/Al2O3 eutectic fiber.