Abstract: Water toughened medium manganese steel demonstrates good workability after annealing treatment, while water retoughening after machining, lead the steel brittle fracture combined with cleavage and inter-granular fracture in room-temperature tensile tests, and considerable drop down in ductility. Research was made on the brittle fracture with methods including fracture morphology, microstructures, X-ray diffraction and surface analysis. The results showed that the inter-granular fracture resulted from aggregation of impurities in grain boundaries, and the cleavage fracture closely related to accruing of large number of inter-granular fractures and existence of deformational twins inside austenite grains. The cleavage plane is the close-packed plane (111) of a F. C. C structure.