The decolorization process of methylene blue (MB), as a model pollutant for dye wastewaters, has been investigated using an LED-irradiated glass microreactor. The main part of the microreactor is a semi-elliptic microchannel with the width, depth and length of 100 microns, 40 microns and 50 cm, respectively. For the purpose of comparisons and exploration of surface effects, glass and polystyrene cuvettes are employed as a model of a batch vessel. It is found that MB was decolorized by UV light and, further, that the photochemical decolorization process was intensified, in terms of reaction rate, by using the microreactor. Especially, at high pH conditions, the process intensification is found to be more prominent due to surface interactions, which are enhanced in the microreactor due to its high specific surface area.