Abstract: The microstructure evolution of medium carbon steel during warm deformation includes dynamic recovery and recrystallization of ferrite, precipitation and spheroidization of cementite. The effect of initial microstructure on the microstructure evolution of a medium carbon steel (0.48% C in mass fraction) during warm deformation was investigated by hot uniaxial compression tests. The results indicate that the lamellar structure of pearlite in the initial microstructure of pearlite + ferrite is kinked, dissolved and spheroidized during warm deformation, with dynamic recovery and dynamic recrystallization of ferrite, the carbides precipitate from supersaturated ferrite, and then the duplex microstructure consisting of sub-micrometer ferrite grains and cementite particles is obtained, while the heterogeneous and zonal distribution of cementite particles and ferrite grains appear along the deformation direction. When the initial microstructure is martensite, the decomposition of martensite and the recovery and recrystallization of ferrite are enhanced by warm deformation. Because of the uniform distribution of carbon in the matrix, the fine equiaxed ferrite grains and homogeneously distributed cementite particles are formed.