Molecular dynamics (MD) simulation of calcium carbonate at high pressure was performed to understand the phase transition between calcite [Formula: see text] and CaCO(3)-II (P 2(1)/c). In the 300-800 K temperature range, the transition of calcite to CaCO(3)-II was reproduced at a pressure of around 8 GPa. This transition is of first order and reversible in the MD calculations except for runs at 300 K where a small hysteresis exists. The slope of the dP/dT curve at the phase boundary between calcite and CaCO(3)-II is negative at 300 K and turns positive at around 600 K, which was confirmed by analyzing the enthalpy change. Just below the transition pressure, the P 2(1)/c structure appears and its orientation switches among three positions with time, resulting in the maintenance of the [Formula: see text] structure as a whole. The P 2(1)/c structure resembles the structure of CaCO(3)-II on an increase of temperature. It can be suggested that the existence of the P 2(1)/c structure and the switching of its orientation just below the transition pressure are responsible for the change of the slope of the dP/dT curve at the boundary from negative to positive on an increase of temperature, because the switching increases entropy and results in an expansion of the stability field of calcite.