Catalysis by CaO/SiO2 composite (silica-supported calcium oxide) was studied for production of biodiesel, which has attracted interest as a means to reduce greenhouse gas emissions. The catalyst composite was prepared by carbonation of Ca(OH)(2) in an aqueous suspension including the silica support, followed by calcination in an argon gas flow. In the carbonation, the silica support could be coated with CaCO3 as precursor of the active material below 18 degrees C. Above 30 degrees C, the precipitated CaCO3 did not coat the silica support at all. The precursor composite with an appreciable core-shell structure prepared below 18 degrees C was subjected to calicimation at 900 degrees C, then the catalytic activity of the composite was fesfecl in the transesterification of soybean oil with methanol at its refluxing temperature. However, the testing reaction was not catalyzed. Since all CaCO3 particles were converted into calcium silicate after the calcination due to reaction with the silica support, CaO as the active material could not survive on the surface of the composite sample. Such the catalytic deactivation was overcome by increasing of the amount of CaCO3 particles on the surface of the silica support. This operation provided CaO survivors on the surface of the improved catalyst composite obtained after the calcination, and the transestrification of soybean oil was approximately completed in 3 h over the improved catalyst composite. The improved catalyst composite was as active in the test reaction as fine particles of bulk CaO.