Esterification reaction between phosphoric acid and methanol proceeded markedly at 6 GPa and 25 degrees C. The yields of monomethyl phosphate and dimethyl phosphate were 17.0% and 0.3%, respectively. The yield of monomethyl phosphate at 6 GPa was at least 34 times higher than that under ambient conditions. The esterification reaction was accompanied by crystallization of ice VII, a high-pressure form of ice. It is likely that the formation of ice VII was the driving force of the esterification reaction.