The effects of grain size on wedge-type crack initiation were investigated under high temperature low-cycle fatigue conditions in an austenitic 304 stainless steel at 973 K. Accumulated grain-boundary sliding (GBS) displacement and the crack initiation life were measured in fatigue tests with slow-slow and slow-fast strain wave-shapes. The accumulated GBS displacement per cycle was proportional to the grain diameter,D.Wedge-type cracks were initiated by the accumulated GBS at grain-boundary triple junctions. The crack initiation life, n1was proportional to D−0.5The grain-size dependence of n1agreed well with the analytical result that was obtained with the micromechanics model analysed by the authors.