It is demonstrated that heat treatment of a LiNbO3plate at temperatures somewhat lower than the Curie point causes a ferroelectric domain inversion, thereby yielding an inversion layer useful for various piezoelectric devices. The dependences of the inversion layer thickness on the heat treatment conditions, such as temperature, time, and atmosphere, are experimentally examined. The results show that the domain boundary appears to stop at the median plane of the plate finally. The possible mechanism of the domain inversion is also discussed.