PURPOSE: To prepare monomethyl terephthalate in high productivity and in high yield, by reacting dimethyl terephthalate with a caustic alkali at a specific ratio in a specific amount of methanol.

CONSTITUTION: (A) Dimethyl terephthalate is reacted with (B) a caustic alkali (potassium hydroxide, etc.) in (C) methanol, wherein the amount of (A) is 0.015W 0.070, preferably 0.025W0.040 mole on the basis of 1 mole of (C) and the amount of (B) is 0.8W1.5, preferably 0.9W1.3 mole on the basis of 1 mole of (A). The feed of the caustic alkali (B) is 0.0007W0.06, preferably 0.002W0.05mol/hr on the basis of 1 mole of methanol (C) in the reaction system. The product is neutralized to give the desired compound. The method suppresses the formation of undesired terephthalic acid, remarkably increases the productivity per unit volume and gives a high yield.