Medium manganese (Mn) steel has excellent strength and ductility and is widely used in the automobile industry and elsewhere; however, hydrogen embrittlement (HE) is considered as one of the most serious issues of its application. In this paper, in situ transmission electron microscopy was used to study the microstructural evolution and fracture micromechanisms of medium Mn steel under the influence of hydrogen and annealing. The results indicated that two specimens with different annealing processes showed transformation-induced plasticity and twinning-induced plasticity, albeit to a varying degree due to different microstructures with distinct mechanical properties. It was shown that hydrogen affected the fracture micromechanisms, which promoted the initiation of microcracks at the boundaries of martensite transformed from austenite. Moreover, the main cracks propagate in a zigzag fashion, following hydrogen charging, due to the hardness of the martensite. These findings provide directions for improving HE resistance through microstructural regulation of medium Mn steel. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.