Heterogeneous photocatalysed reduction of aqueous Na_2CO_3 was carried out by using nano LaCoO_3 semiconductor powders. Formic acid and formaldehyde were identified as photoproducts,and were measured spectrophotometrically using Nash reagent. The effect of different parameters such as sodium carbonate concentration,amount of photocatalyst and different light sources on the yield of photoproducts was also investigated. It shows that nano LaCoO_3 has photocatalytic activity. Irradiation leads to the production of electrons in the conduction band of the LaCoO_3 semiconductor. It is likely that the photoproduced electrons initially reduce CO_3 (2-) to HCOO -,and then to HCHO and CH_3OH. Heterogeneous photocatalysed reduction of aqueous Na_2CO_3 was carried out by using nano LaCoO_3 semiconductor powders. Formic acid and formaldehyde were identified as photoproducts, and were measured spectrophotometrically using Nash reagent. The effect of different parameters such as sodium carbonate concentration, amount of photocatalyst and different light Sources on the yield of photoproducts was also investigated. It shows that nano-LaCoO_3 has photocatalytic activity. It is likely that the photoproduced precursors reduce CO_3 (2-) to HCOO -, and then to HCHO and CH_3OH.