This research investigated the phase transition between low- and high-temperature orthopyroxenes with composition (Ca0.06Mg1.94)Si2O6 using differential scanning calorimetry experiments and in situ high-temperature X-ray diffraction. The transition enthalpy, temperature, volume change, and slope were estimated to be 6.2 kJ/mol, 1170 °C, 10.25 Å3/unit cell, and 0.0056 GPa/°C, respectively. The phase boundary between low- and high-temperature orthopyroxene was defined as P (GPa) = 0.0056T (°C) − 6.55. This relationship shows that the invariant point for four-phase equilibria (protoenstatite + high-temperature orthopyroxene + pigeonite + diopside) is approximately 1240–1280 °C and 0.1–0.2 GPa, rather than the equivalent system involving low-temperature orthopyroxene as described in previous studies. We developed phase diagrams for Mg2Si2O6 and the Mg2Si2O6-CaMgSi2O6 system taking into account the results of previous synthetic experiments and the phase boundary that we determined between low- and high-temperature orthopyroxene. The developed phase diagrams for Mg2Si2O6 showed that high-temperature orthoenstatite is more stable than protoenstatite at pressure above ~0.8 GPa, and that the boundary between high-temperature orthoenstatite and protoenstatite has a gentle negative slope. As pressure is increased from 1 atm to about 0.2 GPa, the lower temperature limit of stability of high-temperature orthopyroxene decreases from ~1370 to ~1200 °C. Above 0.9 GPa, the stability field of protoenstatite disappears and high-temperature Ca-free orthopyroxene is stable. On the basis of these results, it is suggested that further high-resolution analyses of the thermodynamics of the enstatite-diopside system at high temperatures and high pressures are required.