A method for simultaneous treatment of an aqueous solution to photolytically sterilize microbial cells and photocatalytically decompose organic compounds was studied using an annular-flow photocatalytic reactor with a 6-W germicidal lamp (wavelength; 254 nn) as a light source. All the experiment were carried out in a batch recirculation mode. When a thin film of titanium oxide was located on the outer surface of a quartz glass tube, which was set to protect the germicidal lamp from direct contact with an aqueous solution flowing through the annulus, i.e., when the photocatalyst was located at the inner surface of the annulus, the sterilization and decomposition slowed down. This is due to the fact that the degree of transparency of UV lights with wavelengths less 300 nm is significantly deceased by the film of titanium oxide. When the film of titanium oxide was located at the outer surface of the annulus, on the other hand, both the rates of sterilization and decomposition became higher. An experimental result using an aqueous solution containing E. coli and formaldehyde showed that the sterilization off. coli is dependent on the circulation rate of the solution but the photocatalytic decomposition of formaldehyde is independent of it and can be approximated by a perfectly-mixed reactor model. Consequently, it was found that the annular-flow photocatalytic reactor used in the present work makes it possible to simultaneously carry out the photolytic sterilization of microbial cells and the photocatalytic decomposition of organic compounds.