Heterogeneous grain structures were designed in a high Mn steel (Fe28Mn10Al1.00C), and the tensile properties and fracture toughness were investigated and compared with those for homogeneous structures. The heterogeneous grain structures display larger tensile ductility, stronger strain hardening and higher fracture toughness at the similar yield strength level. Hetero-deformation-induced hardening is found to play an important role in the heterogeneous grain structures, resulting in better mechanical properties. The size of plastic zone and the strain hardening capacity around the crack tip for the heterogeneous grain structures are found to be much larger/higher than those for the homogeneous grain structures at the same level of yield strength, resulting in better fracture toughness. High density of geometrically necessary dislocations and grain refinement are induced at the adjacent area of the main crack path, and numerous microvoids are also observed besides the main crack for the heterogeneous grain structures, resulting in more energy dissipation for higher fracture toughness. The deformation mechanisms around the crack tip are highly dependent on the magnitude of plastic strain and the grain size. The observed higher fracture toughness in the heterogeneous grain structures can be partly attributed to the formation of microbands. (c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).