A novel planar photocatalytic microreactor coated with metal ion-doped TiO2 was fabricated by chemical etching and sol-gel techniques. The photocatalytic activity of the microreactor was evaluated by studying the degradation of methylene blue (MB) under ultraviolet light, visible light, and sunlight, and encouraging results were obtained. Analyses of the degradation products indicate that demethylation and ring-opening reactions took place and that the mineralization of MB is a slow process. Kinetic studies of the MB degradation show that metal ion doping increases the reaction order. Comparing the applied microreactor with other conventional microchannel photocatalytic reactors, nearly the same degradation ratios and relatively higher degradation rates per unit reaction volume were obtained.