In our previous study, the photo decolorization of methylene blue (MB) was investigated using a UV-LED-irradiated glass microreactor without photocatalyst. The main part of the microreactor was a semi-elliptic microchannel with the width, depth and length of 100 mu m, 40 mu m and 50 cm, respectively. In this study, as an extension of the previous work, we investigated the decolorization behavior of MB in the same type of reactor, on the inner wall of which a photocatalyst (TiO2) is immobilized. It was found that the decolorization was much increased by the presence of the photocatalytic layer, while decolorization slightly occurred in its absence, and even without UV irradiation. At neutral and high pH conditions, the process required an extremely long time to reach the adsorption equilibrium of MB, probably due to the increased affinity between MB and catalyst surface. An increase in feed flow rate caused fluctuating adsorption behavior after long operation, most likely due to the high shear stress in the microchannel. It was thus suggested that the performance of photocatalytic microreactors should be evaluated by taking into account the long time necessary for achieving adsorption equilibrium if the model pollutant is MB.