Removal of naphthenic acid in model acid solution by the catalytic decarboxylation using alkaline earth metal oxide catalysts such as MgO, CaO, BaO and SrO was investigated. Among the alkaline earth metal oxides, MgO and CaO showed better activity than BaO and SrO. The surface properties of MgO have been modified by calcination or hydration followed by dehydration under vacuum and the effects of those treatments on the catalytic activities were investigated. Although the calcination at higher temperature reduced the BET surface area and number of basic sites, it gave the highest decarboxylation activity. The CP-MgO which was made by hydration-dehydration under vacuum gave the highest BET surface area and number of basic sites. But, the catalytic activity of CP-MgO was inferior to the catalyst calcined at 800 ℃. This fact implies that both pyrolytic and catalytic reactions play important roles in decarboxylation of naphthenic acid. FT-IR spectra of the catalysts before and after reaction gave the evidence for the formation of bulk magnesium naphthenate, which is the reaction intermediate of the pyrolytic decarboxylation.