This invention relates to the preparation of optically pure tetrahydro-3-isoquinolinecarboxylic acid derivative of formula (III) which comprises the reaction of optically pure phenylalanine derivative of formula (I) with a formaldehyde precursor, such as formaline, trioxane, dialkoxymethane, or paraformaldehyde, for about 10 hours to 60 hours at about 40 .degree. C. to about 60 .degree. C. in concentrated hydrochloric acid to give the compound of formula (II), followed by neutralization in hot water at about 70 .degree. C. to about 100 .degree. C. with a base, such as ammonium hydroxide, potassium carbonate, sodium carbonate, sodium hydroxide or potassium hydroxide, according to the following reaction scheme. ##STR1##