Large single crystals of MgSiO3 perovskite were successfully synthesized by a thermal gradient method at 24 GPa and 1500 °C. This was achieved by an improvement of high-pressure synthesis technique that allowed us to grow single crystals under such ultrahigh-pressure conditions in relatively large volumes (e.g., 10 mm3). Since crystal growth is hindered by neighboring crystals, the nucleation density was suppressed by reducing the thermal gradient to 20 °C/mm, permitting an increase in free space for large crystal growth. KHCO3-Mg(OH)2 solvent can be used to grow perovskite crystals. However, the carbonate solvent produces melt inclusions. Silicate sources with MgSiO3 composition produce stishovite inclusions, which in turn cause splitting of perovskite crystals. The formation of these inclusions is avoided by using H2O as a solvent and 85MgSiO3-15Mg2SiO4 as a silicate source. The H2O also allows homogeneous crystal growth, probably because of its low viscosity and high silicate solubility. High-quality single crystals larger than 1 mm were successfully synthesized through these technical developments.