A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of single-crystal superalloys used in aircraft engine turbine blades. In this development, a commercial ultrasonic fatigue testing machine was considerably modified to achieve high-temperature fatigue testing. The developed system took account of temperature dependency of Young's modulus, and also had a function to evaluate the Young's modulus. In order to protect the testing system from the heat of a specimen, straight and round rods were interpolated between the testing system and the specimen. Other modifications achieved accurate control of temperature, edge displacement and resonance frequency, which were necessary for accurate control of stress amplitude. The testing system was first applied to a heat-resistant steel at 650 °C to check its accuracy, and next to single-crystal superalloy samples at 1000 °C. As a result, the ultrasonic fatigue testing showed good agreement with the conventional fatigue testing, demonstrating the high accuracy of the developed system. In these results, the single-crystal superalloys showed no fatigue limit, indicating gigacycle fatigue tests to be necessary.