A novel fabrication of Cu 2 O nanoparticles was successfully achieved through the reduction of Cu 2+ in the interlayer of montmorillonite (denoted as MT). The Cu 2 O nanoparticles formed in the clay interlayer (denoted as Cu 2 O/MT) had a lamellate shape of 20–100nm in width and ca. 0.39nm in thickness, and thus, the Cu 2 O/MT exhibited a higher specific surface area than neat Cu 2 O. The Cu 2 O/MT was applied to a photocatalyst, and its photocatalytic activity was examined in terms of water reduction (i.e., H 2 evolution) in the presence of methanol (electron donor). The Cu 2 O/MT showed a high photocatalytic activity in comparison with neat Cu 2 O; moreover, the photocatalytic activity was improved by loading a Pt catalyst onto the surface of Cu 2 O. Based on the dependencies of photocatalytic activity on light intensity as well as methanol concentration, it was revealed that the H 2 evolution from water on Cu 2 O/MT is the rate-determining step. Thus, the active and efficient photocatalysis of Cu 2 O/MT was associated with an increase in specific surface area corresponding to the number of active sites.