(Mg, Fe)SiO 3 orthopyroxene is an abundant mineral of oceanic subducting slabs. In-situ high-pressure and high-temperature single-crystal X-ray diffraction has been used to investigate the phase transition of orthopyroxene across the enstatite-ferrosilite (En-Fs) join (En 70 Fs 30 , En 55 Fs 45 , En 44 Fs 56 and Fs 100) up to 24.3 GPa and 800 K, simulating conditions within the coldest part of a subduction zone consisting of an old and rapidly subducting slab. Instead of the orthopyroxene → high-pressure clinopyroxene transition, the α-opx → β-opx and β-opx → γ-opx phase transition are observed at 7.2-15.3 and 11.6-21.1 GPa (depending on the Fs content), respectively. This study indicates that the pressure-induced phase transition of (Mg, Fe)SiO 3 orthopyroxene under relatively low temperature (<800 K) could be different than those occurring under relatively high temperature (>800 K). Additionally, the α-opx → β-opx → γ-opx phase transition could exist within the center of the extremely cold slabs (like Tonga), where such low temperature persists to~600-km depth. Plain Language Summary (Mg, Fe)SiO 3 is the most abundant chemical compound in Earth's mantle. Investigating its phase relations at high-pressure and high-temperature is of fundamental importance in constraining the composition and understanding the structure of the Earth's interior. Previous studies have well established the phase diagram of (Mg, Fe)SiO 3 at high pressure and high temperature. Orthopyroxene is a low-pressure phase; with increasing pressure and temperature, orthopyroxene transforms to the high-pressure clinopyroxene, then, it transforms to akimotoite or decomposes to wadsleyite + stishovite. These phase transitions require temperatures higher than 900 K, which are easily satisfied in the normal mantle or relatively warm subduction zone conditions. However, extremely low-temperature region exists in the center of some old and rapidly subducting slabs (e.g., Tonga), and such low-temperature region (~800 K) can extend to~600-km depth. In this study, we investigate the phase relations of (Mg, Fe)SiO 3 orthopyroxene with several different compositions to 24.3 GPa and 800 K. A different phase transition path is observed, that is, the α-opx → β-opx → γ-opx at pressures within the transition zone depths. Therefore, the result of this study adds one more piece of information to our understanding of phase relations of (Mg, Fe)SiO 3 orthopyroxene within the extremely cold subducted slabs.