The factors in the catalytic dehydration of carboxylic acid to form ketene on silica were studied. Many research efforts have been concentrated on the ketene synthesis using low molecular weight carboxylic acid(C2-C4). However, there is no report so far on the ketene synthesis using high molecular weight carboxylic acid, and the reason may be attributed to the difficulties in controlling the reaction condition and analysis. We carried out ketene synthesis in vacuum using high molecular weight carboxylic acid as a reactant and checked the factors which affect catalytic activity. The concentration of silanol was decreased by raising the pretreatment temperature of the catalysts, and was proportional to the catalytic activity. Consequently, it was confirmed that surface silanol was the active site. By changing the thickness of the catalyst bed and the degree of vacuum, the short contact time of the reaction also was confirmed to play an important role in getting higher ketene yield.