Dimerization of methyl crotonate was studied for active solid base catalysts as well as for elucidation of the reaction mechanisms on the solid base catalysts. Among various types of solid base catalysts, MgO showed a higher activity than those of the other solid base catalysts, such as CaO, SrO, BaO, ZrO2, La2O3, KF/alumina, KOH/alumina and K-X zeolite. Dimerization of methyl crotonate proceeds over MgO by Michael addition, which is initiated by abstraction of an allylic hydrogen of methyl crotonate by the basic site on the catalyst to form allylic carbanion, The anion attacks a second methyl crotonate molecule at the beta-position to form a methyl diester of 3-methyl-2-vinylglutaric acid which undergoes double bond migration to form the final products : the methyl diesters of (E)-2-ethylidene-3-methylglutaric acid and methyl diesters of (Z)-2-ethylidene-3-methylglutaric acid. The active sites of MgO are discussed on the basis of the product distribution that varies with the pretreatment temperature of the catalyst.