β-Naphthol is the most important dye intermediaters in the preparation of azo dyes, and is made by the alkali fusion of β-naphthalene sulfonic acid with sodium hydroxide. This study on sulfonation aimed at the production of β-naphthalene sulfonic acid and carried out in varying temperatures between 130°-160℃ for about 8 hrs, where the initial mole ratio of sulfonic acid vs to naphthalene was used 1:1. Sampling was taken by the time intervals of 0.5 or 1 hr. The unreacted sulfonic acid in the sample was analyzed by titration with M/100 EDTA and both of α-and β-sulfonic acid which was produced during the reaction was analyzed by using "Euwes`s solubility chart of Pb salt". From the result of this experiment, it was found that this reaction was nothing but a first order consecutive reversible one though this reaction was expressed as follows, "naphthalene+sulfonic acid★α-naphthalene sulfonic acid+water★β-naphthalene sulfonic acid+water"