The effects of thermal cycling on martensite transformation in Cu 11.9Al 2.5Mn(mass fraction, %) shape memory alloy(SMA) with high M s temperature were studied by means of electrical tension vs temperature ( U-T ) measurement, optical microscopy observation and X ray diffractometry. It shows that with increasing thermal cycles, the transformation temperatures decreased, which are accompanied by changes of martensite structure such as the decline of NNN ordering degree and monoclinic angle β →90°. Compared with traditional Cu base SMA, the alloy has rather high thermostability and can work at 200～300?℃. Its excellent thermostability comes from two factors: 1) its β 1 parent structure is stable and difficult to decompose at work temperature; 2) its martensite structure is close to N18R( β =89.6°), which restrains the process of M18R→N18R and the tendency of martensite stabilization and decomposition. The effects of thermal cycling on martensite transformation in Cu 11.9Al 2.5Mn (mass fraction,%) shape memory alloy (SMA) with high M s temperature were studied by means of electrical tension vs temperature (UT) measurement, optical microscopy observation and X ray diffractometry. It shows that with increasing thermal cycles, which are accompanied by changes of martensite structure such as the decline of NNN ordering degree and monoclinic angle β → 90 °. Compared with traditional Cu base SMA, the alloy has Its superior thermostability comes from two factors: 1) its β 1 parent structure is stable and difficult to decompose at work temperature; 2) its martensite structure is close to N18R (β = 89.6 °), which restrains the process of M18R → N18R and the tendency of martensite stabilization and decomposition.