The effect of high-frequency cycling (120 Hz) superimposed on high tensile mean stress on a lifetime of cast Ni-base superalloys IN 713LC at 800 °C and MAR-M 247 at 900 °C was investigated. It has been found that both the alloys exhibit qualitatively similar behavior in creep/fatigue loading conditions. Superposition of small stress amplitudes on the tensile mean stress does not influence the time to fracture until the stress amplitude reaches a threshold value, which is dependent on the mean stress. The higher the mean stress the higher is the stress amplitude threshold value. The observed creep/fatigue behavior is governed by two mechanisms: (a) formation and growth of intergranular cracks by creep mechanism (b) initiation and growth of fatigue cracks from the intergranular creep cracks. The casting defects do not play any important role as regards the lifetime under investigated loading conditions.