In-situ experiments on the Fe/Al2O3 interface reaction were carried out with a high temperature X-ray diffractometer capable of measuring the X-ray diffraction pattern in 1-4s, using an imaging plate. The kinetic formation processes of the interface reaction layer were measured in short-period exposure experiments using the apparatus. The time-temperature phase diagram of Fe/Al2O3 in air was determined. FeAl2O4 was formed at the Fe/Al2O3 interface between 1595 K and 1675 K in air. The formation of FeAl2O4 obeyed the parabolic rate law. The value of the activation energy suggests that the diffusion of Al into FeAl2O4 controls the rate of formation. The results of thermal expansion coefficient measurements suggest that when a sample is cooled to room temperature, compressive strain caused by FeAl2O4 occurs on Al2O3