A low cycle bending fatigue test was carried out at 700°C with two types of strain wave forms, a symmetrical triangular-type and a one-side hold trapezoidal-type, using specimens with a weak grain boundary (WQ specimens) and specimens with a strong grain boundary (FA specimens) of an austenitic heat-resisting steel. In symmetrical triangular strain wave form, transgranular fatigue cracks were found to initiate and propagate to final fracture in both specimens, so that their fatigue lives were nearly equal irrespective of grain boundary strength. It was found in one-side hold trapezoidal-type strain wave form that WQ specimens fractured due to intergranular creep cracking from the side of tension holding, while FA specimens fractured due to transgranular fatigue cracking from the side of compression holding because of its strong grain boundary. This fatigue fracture process resulted in a much longer life in the FA specimens than that in the WQ specimens.