Abstract: Grain size and magnesium effects on stress rupture and notched cyclic stress rupture properties have been investigated.A minute amount addition of Mg and grain refinement can successfully improve high temperature stress rupture notch sensitivity.The failure lives of iron-nickel base superalloy GH169 with high temperature stress rupture notch sensitivity or low stress rupture ductility decrease dramatically at creep and fati-que interaction conditions. The notched cyclic stress rupture lives are emarkably enhanced by addition microalloying of magnesium and grain refinement even the presence of duplex structure with certain of small grains and large grains.