To produce o-alkylphenols useful as a raw material, etc., for antioxidants or perfumes by readily removing unnecessary residues.

Phenols are reacted with olefins in the presence of aluminum phenolates (with an aluminum content of 0.001-0.5 pt.wt. based on 100 pts.wt. phenols) at 40-150°C under a pressure while keeping the water concentration in a liquid layer in the reactional system at ≤350ppm to afford the objective o-alkylphenols, e.g. 2,6-ditert-butylphenol. Even when the aluminum phenolates are decomposed and deactivated by adding a deactivator (e.g. water) thereto during the reaction, a fluid liquid is obtained and then readily removed without producing infusible and insoluble solid substances derived from the aluminum component in decomposition residues. As a result, the solid substances are not deposited on a heater tube wall, etc., of a distilling plant.