The Clapeyron slope of the perovskite to post-perovskite phase transition has not been well determined experimentally. Crystal structure of CaIrO₃ is equivalent to the MgSiO₃ post-perovskite phase (space group: Cmcm) at ambient condition. We have found that perovskite phase (Pbnm) of CaIrO₃, which is isostructural with MgSiO₃ perovskite, is stable at 1–3 GPa and 1400–1550°C. The perovskite to post-perovskite phase transition in CaIrO₃ was determined by using piston-cylinder apparatus at 0.9–3.0 GPa and 1350–1550°C. Results show that phase transition pressure is strongly temperature-sensitive with a Clapeyron slope of +17(±3) MPa/°C. These results, obtained for a suitable analogue material, suggest that post-perovskite phase transition in MgSiO₃ also has a large positive value of the Clapeyron slope. Such a pronounced exothermic reaction in the Earth's lowermost mantle will promote the upwelling of high-temperature plumes from the core-mantle boundary region.