Cyanobacteria were inactivated by using zinc oxide (ZnO)coated on the surface of gamma-AI2O3 as a photocatalyst and combining with sunlight. In vitro experiments indicate that axenic cultures of planktonic cyanobacteria lost their photosynthetic activity after photocatalyzed with sunlight exposure exceeding 24 hr. As for Oscillatoria tenuisa, nearly 92% of the cells lost their photosynthetic activity and the cell morphology was severely damaged during 24 hr of the reaction. However, in the case of Microcystis aeruginosa, lower photocatalytic inactivity efficiency was observed, which was attributed to extracellular polymeric secretions (EPSs) surrounding the cells. With a high dosage ZnO catalyst, the dissolved organic carbon (DOC) concentration of the Oscillatoria tenuisa suspension was increased by up to about 190% during photocatalysis. The increased suspension of DOC was attributed to increase liberation of extracellular organic and cell-wall polysaccharides during photocatalysis.