The effect of the metal oxide catalyst in the dimerization of waste vegetable oil was investigated. The high efficiencyand recyclability has allowed different metal oxides to be used as catalysts in numerous synthetic reactions. Herein, clay,aluminum, titanium, calcium, magnesium and silicon oxide micro/nanoparticles are used in a Diels-Alder reaction tocatalyze the production of the dimer acids. The metal oxides assist the electron transfers during cyclization to producethe desired product. Liquid chromatography mass spectroscopy (LC-MS) and gel permeation chromatography (GPC) wereused to verify the production of dimer acids. For the confirmation of cyclization, compounds were analyzed using thenuclear magnetic resonance (NMR) spectroscopy. From the analysis, silylated or pristine clay showed its effectivenessas a catalyst in dimerization. Furthermore, alumina and alumina/silica composite showed successful performance in thereaction to yield cyclic dimer acids. These result suggested that metal oxides and montmorillonite might be used insynthesis of dimer acids for the recycle of waste vegetable oils.