This study aims to elucidate the correlation between the carbide ratio and the hardness of high-manganese (Mn) steel treated under various heat treatment conditions. The content of the Mn and carbon (C) in the used steel was controlled at 9.15 and 0.94 wt.%, respectively. After being melted in the muffle furnace, the as-cast steel was annealed in the 800–1100 °C range using a resistant furnace and then cooled to room temperature in different ways. The microstructure was examined, and the carbide ratio was calculated based on optical micrographs. The obtained results showed that the increased temperature reduced the carbide ratio from 33 to 6% for the steel cooled in the furnace and from 23 to 4% for the one cooled in air, causing a decrease in hardness from 66 to 56 HRA and from 62 to 57 HRA, respectively. The carbide ratio in the present steel was attributed to the change in the steel’s hardness. It was found that the proper annealing temperature must be around 1100 °C to get a compromise of the steel’s carbide ratio and hardness for the application.