We report a detailed study of steplike magnetization and resistivity jumps in a bilayered (La0.5Nd0.5)1.2Sr1.8Mn2O7 single crystal. The sample exhibits very sharp metamagnetic transitions at low temperature when the magnetic field is applied either in an ab plane or along the c axis, which causes huge magnetization steps. The critical field depends on the cooling magnetic field as well as the sweep rate of the magnetic field. Meanwhile, the evolution with time of the magnetization exhibits a spontaneous step when both the temperature and magnetic field are constant. Similar steplike behaviors are also observed in resistivity. These results suggest that a martensiticlike transformation could happen in bilayered manganites.