The crystal structure and thermal stability of the martensite in Cu 24Al 3Mn (mole fraction, %) alloy were studied by X ray diffraction, electron diffraction, V - t curve measurement. The 18R martensite with fairly perfect long range order can be obtained by water quenching the alloy. The atoms distribution on the basal plane of the martensite is as follows: Ⅰ, 3/25 Mn+22/25 Cu; Ⅱ, 3/25 Al+22/25 Cu; Ⅲ, 18/25 Al+7/25 Cu, correspondingly, the parent phase may have Heusler structure. From the electron diffraction result, the crystal structure is determined to be of a modified 18R type but closely approaching orthorhombic 18R type, its lattice parameters are determined to be a = 0.447?4?nm, b = 0.522?9?nm, c = 3.815?nm and β =89.6° from the X ray diffraction results. The obtained alloy has a higher thermal stability than that of the conventional Cu Zn Al alloy. The crystal structure and thermal stability of the martensite in Cu 24Al 3Mn (mole fraction,%) alloy were studied by X ray diffraction, electron diffraction, V - t curve measurement. The 18R martensite with fairly perfect long range order can be obtained by water quenching the alloy. The distribution of atoms on the basal plane of the martensite is as follows: Ⅰ, 3/25 Mn + 22/25 Cu; Ⅱ, 3/25 Al + 22/25 Cu; Ⅲ, 18/25 Al + 7 / 25 Cu, correspondingly, the parent phase may have Heusler structure. From the electron diffraction result, the crystal structure is determined to be a modified 18R type but closely approaching orthorhombic 18R type, its lattice parameters are determined to be a = 0.447? 4? Nm, b = 0.522-9 nm, c = 3.815 nm and? = 89.6 ° from the X ray diffraction results. The obtained alloy has a higher thermal stability than that of the conventional Cu Zn Al alloy.