The evolution of fracture mode was investigated in Fe-1.1C-12.5Mn steel with prior introduced nano-twinned structures. The thicknesses of the prior twin plates were manipulated by thermo-mechanical treatments. During the plastic deformation, thin prior twin plates (30–40 nm) were penetrated by dislocations which activated secondary twin systems, and therefore dimple features were promoted in the fractured surfaces due to dislocation accommodation within the nano-twin plates. However, dislocation pile-up on the boundaries of the thick prior twin plates (90 nm) led to trans-granular fracture and formation of cracks. Moreover, prior twin elimination enhanced inter-granular feature appearance in the fractured surfaces because of dislocation accumulation on grain boundaries.