The shape memory effect exhibited by Fe-Mn-Si based alloys is due to the γ(f.c.c.)-ε(h.c.p.) martensitic transformation. The martensitic transformation induced by traction at room temperature in a Fe-16Mn-9Cr-5Si-4Ni (%mass) is here studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunnelling microscopy. The martensitic microstructure and the fine structure of the faults stackings are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out. Inside a grain, the monopartial nature of the martensite have been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.