Abstract: A simple combined fatigue test facility is introduced, which produces and controls high- and low-frequency combined load spectra by means of a closed circuit consisting of a microcomputer and a hydraulic servo unit. The minor-cycle frequency attains 100Hz with a force amplitude of about 500kg,but higher frequency(say, 400-500Hz) may be attained if a better servo unit be available.About 50 specimens of aluminum alloy LY12-CZ have been tested and the results show that both the minor-cycle frequency and force amplitude have a detri-mental effect on crack initiation life.A ratchetting phenomenon appeared as the minor-cycle frequency was lowered to l0Hz. It is probably a "deformation failure" in this case.