Diffusionless tetragonal (t′) cubic (c′) phase transformation in 65‐mol%‐CeO2‐ZrO2 was investigated around the c′‐t′ equilibrium temperature T0c′‐t′ using powder X‐ray diffraction, where c′ was defined as a tetragonal or cubic phase with an axial ratio of unity. The {400} peak profile of t′‐ZrO2 broadened at 860° and 900°C. This indicated that the t′‐ZrO2 transformed partially into c′‐ZrO2. The {400} peak profile of c′‐ZrO2 also broadened at 860° and 900°C, which indicated that the c′‐ZrO2 partially transformed to t′‐ZrO2. The finish point of t′→c′ transformation Tft′→c′ was investigated by annealing the t′‐ZrO2 containing 30–65 mol% CeO2 at various temperatures. The T′ft′→c′ line existed in the vicinity of the cubic solubility limit within the t + c two‐phase region. This Tft′→c′ location, which should have been located in the vicinity of T0c′‐t′ could not be explained by a simple regular solution model. However, it was described successfully by a thermodynamic model based on Landau's phenomenologic theory.