The stacking order and morphology of Ti50(Ptx,Ir50-x) martensite were investigated to elucidate the compositional dependence of mechanical properties. Microstructure and stacking sequence were analyzed by TEM, SAED, HRTEM and HAADF-STEM. Observations by (S)TEM revealed each alloy to show different stacking orders: 4H, 6H and 7R for Ti50Pt 12.5lr37.5, Ti50Pt37.5Ir 12.5 and Ti50Pt25Ir25 alloy, respectively. The twinning microstructure also varied according to alloy composition. Ti50Pt12.5lr37.5 alloy is covered with two types of twin, while there are very few twins in Ti50Pt 37.5lr12.5 alloy.