Propene oxidation was carried out with an electrochemical reactor, MoO3/AuYSZAg (YSZ : 8 mol.-% yttria-doped ZrO2), at 475 degrees C under oxygen pumping, and the catalytic activity of the thin MoO3 film related to its crystal morphologies was discussed. The thin MoO3 film was deposited on an Au anode by means of vacuum deposition or sputtering method at room temperature or 300 degrees C. Each method skewed a characteristic texture as well as crystal morphology of the MoO3 film, resulting in variations in the catalytic activity. Sputtering at 300 degrees C gave porous films composed of leaf-like crystals with preferential orientation of (010) plane parallel to the pore channel and perpendicular to the Au surface, resulting in the highest activity for acrylaldehyde production. A relatively high step density was observed on the oriented (010) plane of the leaf-like crystal. The high activity of this MoO3 film is probably due to the high density of active sites for the partial oxidation of propene and also to their highly porous structure, which is favorable to the surface migration of oxygen to the reduced active sites.