The microstructural evolutions of 5 Mn steel during various heat treatments have been investigated by in-situ transmission electron microscopy(TEM).The specimen of 5 Mn steel was prepared using focused ion beam(FIB)milling,which allowed the selection of specific morphology of interest prior to the in-situ observation.The complete austenization at 800°C was verified at the atomic scale by minimizing thermal expansion and sample drift in a heating holder based on micro-electro-mechanical-systems.During annealing at 650°C,the formation of reverted austenite was dynamically observed,while the morphologies of austenite laths of 5 Mn steel after in-situ heating were quite similar to that after ex-situ intercritical annealing.During annealing at 500°C,the morphological evolution of cementite and associated Mn diffusion were investigated.It was demonstrated that a combination of FIB sampling and high temperature in-situ TEM enables us to probe the morphological evolution and elemental diffusion of specific areas of interest in steel at high spatial resolution. The microstructural evolutions of 5 Mn steel during various heat treatments have been investigated by in-situ transmission electron microscopy (TEM). The specimen of 5 Mn steel was prepared using focused ion beam (FIB) milling, which allowed the selection of specific morphology of interest prior to the in-situ observation. The complete austenization at 800 ° C was verified at the atomic scale by minimizing thermal expansion and sample drift in a heating holder based on micro-electro-mechanical-systems. Temperature annealing at 650 ° C, the formation of reverted austenite was dynamically observed, while the morphologies of austenite laths of 5 Mn steel after in-situ heating were quite similar to that after ex-situ intercritical annealing. Fluid annealing at 500 ° C, the morphological evolution of cementite and associated It was demonstrated that a combination of FIB sampling and high temperature in-situ TEM enables us to probe the morphological evolution and elemental diffu sion of specific areas of interest in steel at high spatial resolution.