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, Ac-1 and Ac-3, were 745 and 780 A degrees C, respectively. M23C6 decomposition took place at 820-850 A degrees C and M7C3 at 1060-1130 A degrees 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.