Photocatalytic reduction of carbon dioxide under visible light irradiation was carried out with Co3O4 powders suspended in water. A Pyrex glass batch reactor of 10mL in volume was used with a 21 W LED lamp of 510 to 620 nm in wave length as light source, and the reaction time was held at 4 h. The major products were formic acid and formaldehyde; the production rates were 4.53 μmol g.1h.1 and 0.62 μmol g.1h.1 for formic acid and formaldehyde, respectively. Carbon monoxide and methane were detected in trace amounts. The occurrence of the photo-reduction with Co3O4 is against the expectation from the valence band edge of Co3O4 in the literature. Possible causes for the contradictory result are discussed.