The cementite formation and coarsening behaviors of 0. 2 mass% C-5 mass% Mn steel during tempering at 500℃ were investigated by in-situ transmission electron microscope( TEM). In-situ TEM observation showed uniform distribution of cementite particles at the early stage of tempering in the rapidly heated( 500 ℃ / s) sample. Elemental analysis confirmed that the cementite growth was dominated by Mn diffusion. During the cementite growth,the coarsening behavior of intragranular cementite was significantly controlled by the matrix diffusion,while that of the intergranular cementite was mainly governed by the boundary diffusion. The in-situ TEM observation revealed that the dislocation pipe diffusion of Mn took place during tempering,which accelerated the Mn diffusion between cementite particles. The coarsening rates of individual cementite particles were calculated based on the in-situ TEM observation. The cementite formation and coarsening behaviors of 0.2 mass% C-5 mass% Mn steel during tempering at 500 ° C. were investigated by in-situ transmission electron microscope (TEM). In-situ TEM observation showed uniform distribution of cementite particles at the early stage of tempering in the rapidly heated (500 ° C / s) sample. Elemental analysis confirmed that the cementite growth was dominated by Mn diffusion. During the cementite growth, the coarsening behavior of intragranular cementite was significantly controlled by the matrix diffusion, while that of the intergranular cementite was mainly governed by the boundary diffusion. The in-situ TEM observation revealed that the dislocation pipe diffusion of Mn took place during tempering, which accelerated the Mn diffusion between cementite particles. The coarsening rates of individual cementite particles were calculated based on the in-situ TEM observation.