Photocatalytic oxidation of p-chlorophenol and toluene under gas-liquid-solid multiphase flow conditions was investigated by using a photocatalytic microreaction system. By loading both gaseous and liquid samples simultaneously into a microchannel with a photocatalytic titanium dioxide thin layer therein, a gas-liquid-solid multiphase annular flow was generated. The reaction yield was greatly enhanced with decreasing thickness of liquid layer because of improved efficiency of interaction and mass transfer between different phases.