Abstract: In this paper, the titration curve of potassium naphthenate is acquired by using strong acid for titrating mixed base. The tirating result shows that naphthenic acid (NPA) is a monoprotic weak acid and NPA exists in the type of monomolecular in aqueous solution, compared with the measuring result of molecular weight of NPA. According to the above result, the dissociation constant and the species distribu-tion of NPA in aqueous solution is derived.