Catalytic dehydration of caarboxylic acid on mesoporous silica was investigated. Direct conversion of high molecular weight carboxylic acid to ketene and ketene derivatives was observed. Besides the alkyl ketene dimer (AKD), stearic anhydride and β-keto acid were also formed. AKD is so reactive that it can easily react with water to become ketone via β-keto acid as an intermediate. However, among the catalytic dehydration products, β-keto acid was major product with high yield, but ketone was not detected. In the catalytic dehydration condition, β-keto acid was made from the ring opening of AKD mainly by silanol group. It was the most important key factor to shorten the contact time in catalyst layer for raising the yield of AKD by preventing the formation of β-keto acid.