Thermal stress analysis within lump coke was studied to investigate macrocrack formation and deformation behaviour, which strongly influence heat and mass transfer in a coke oven chamber. The dilatation of the plastic layer, heating-rate dependence of thermophysical and mechanical properties of the coal/coke, creep in the plastic and semicoke layers, macrocrack propagation and radiative heat transfer within the macrocrack were considered in an analytical model. Macrocrack propagation was determined from the estimated stress intensity factor, KI, at the macrocrack tip compared with the plane strain fracture toughness, K-IC, through unsteady-state calculation. Calculated results for crack formation and deformation behaviour of lump coke were in good agreement with experimental observations using a laboratory-scale oven chamber. The analytical model could also predict microcrack formation within lump coke normal to the heated wall and the coke surface close to the heated wall.