The mechanism of catalytic addition reaction of maleic anhydride to castor fatty acid methyl esters by flow method was investigated. It was found that the mono ester of maleic acid with the hydroxyl group of methyl ricinoleate, 1-hexyl-11-methoxycarbonyl-3-undecenyl hydrogen maleate, was formed in 40% yield at 130°C by continuously passing the mixed solution of methyl ricinoleate and maleic anhydride at molar ratio of 1 : 1 over a high surface-area synthetic silica-alumina catalyst composed of 70% of SiO₂ and 29% of Al₂O₃. Then, it was assumed that the mono ester plays a role as intermediate in the dehydration reaction of methyl ricinoleate for producing methyl octadecadienoate.