Medium manganese steel (Fe-5.0%Mn-1.2%Si-0.10%C alloy) was subjected to interrupted quenching from the austenite single-phase region to a temperature between M_s and M_f followed by intercritical annealing in the ferrite and austenite dual-phase region at 923 K. As a result, a core-shell type second phase, which consisted of a fresh martensite core surrounded by a film-like retained austenite shell, was formed. The mechanism and kinetics of reversion for the interrupted-quenched specimens were analyzed with DICTRA simulation and TEM observation. With regard to the effect of the core-shell type second phase on mechanical properties, it was inferred that the fresh martensite core functioned as a hard second phase and enhanced work hardening by stress partitioning similar to DP steel, while the film-like retained austenite contributed to improved ductility due to the TRIP effect. As the interrupted quenching temperature decreased, the volume fraction of the fresh martensite core decreased, while the stability of the retained austenite shell increased. This showed potential for controlling the strength and ductility balance of medium manganese steel. A possible beneficial effect of the core-shell type second phase on the ductile fracture behavior was also discussed in terms of stress/strain relaxation at the interfaces between hard martensite and ferrite matrix.