Styrene was oxidized by molecular oxygen over TiO2/SiO2 for the formation of benzaldehyde. At 70°C and 30 atm air with a styrene flow of 9.0 ml/h and air flow of 120 ml/min over 10.00 g catalyst, styrene conversion of more than 40% with benzaldehyde selectivity of more than 97% was obtained. Formaldehyde, phenylacetaldehyde, acetophenone, styrene oxide, benzoic acid, and minor polymer were found to be the by-products. The polymerization of styrene was initiated by the radicals formed in the oxidative reaction. The addition of radical inhibitor nitrobenzene and/or the employment of a catalyst of high specific surface area can promote the termination of the radicals, and hence improve the selectivity of benzaldehyde.