The characteristics of carbide transformation in an 8 % Cr roller steel under the conditions of equilibrium, slow heating, and isothermal treatment were investigated using thermodynamic calculation, differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. The carbides under equilibrium conditions were identified as MC, M23C6, and M7C3 types. The austenite transformation onset and finish temperatures, Ac1 and Ac3, were 745 and 780 °C, respectively. M23C6 decomposition took place at 820–850 °C and M7C3 at 1060–1130 °C, under slow heating conditions. Quantitative relationships were obtained between the isothermal conditions and average carbide size and aspect ratio, which increased with increasing holding temperature and time.