The synthesis of LaCrO3 from the mixture of chromium hydroxide and La2O3 was studied under hydrothermal conditions and in air. LaCrO3 was directly prepared under hydrothermal conditions of above 400°C under 100MPa H2O pressure in contrast to the formation of LaCrO3 via La2CrO6 above 700°C in air. LaCrO3 synthesized under hydrothermal conditions was well crystallized with homogeneous grain size of 0.7±0.2μm at temperatures of 700°-1000°C, while that synthesized in air was an aggregate with irregular grains.