Molecular dynamics (MD) simulation of calcite was carried out with the interatomic potential model based on ab initio calculations to elucidate the phase relations for calcite polymorphs and the mechanism of the rotational order-disorder transition of calcite at high temperature at the atomic scale. From runs of MD calculations with increasing temperature within a pressure range of 1 atm and 2 GPa, the transition of calcite with [Formula: see text] symmetry into a high-temperature phase with [Formula: see text] symmetry was reproduced. In the high-temperature [Formula: see text] phase, CO(3) groups vibrate with large amplitudes either around the original positions in the [Formula: see text] structure or around other positions rotated ± 60°, and their positions change continuously with time. Moreover, contrary to the suggestion of previous investigators, the motion of CO(3) groups is not two-dimensional. At 1 atm, the transition between [Formula: see text] and [Formula: see text] is first order in character. Upon increasing temperature at high pressure, however, first a first-order isosymmetric phase transition between the [Formula: see text] phases occurs, which corresponds to the start of ± 120° flipping of CO(3) groups. Then, at higher temperatures, the transition of [Formula: see text] to [Formula: see text] phases happens, which can be considered second order. This set of two types of transitions at elevated pressure can be characterized by the appearance of an 'intermediate' [Formula: see text] phase between the stable region of calcite and the high-temperature [Formula: see text] phase, which may correspond to the CaCO(3)-IV phase.