Photocatalytic oxidation of free cyanide (CN-) in water directly using solar energy is highly desired. However, most of the photocatalysts with effective performance are solely metal-based semiconductors. Herein, the metalfree graphitic carbon nitride nanosheets (g-C3N4-NS) have been prepared and utilized as photocatalysts for oxidation of CN- under visible light. The removal of CN- over g-C3N4-NS can reach 90% after 150 min when the initial concentration of CN- is 0.18 mM and the cyanate (CNO-) is the only product. The ESR and quenching experiment results reveal that the O2(center dot-) and (1)O(2 )play significant roles in the photocatalytic oxidation of CN- : first, the oxygen can be reduced by the photogenerated electrons (e(-)) from g-C3N4-NS to generate O2(center dot-)O2(center dot-) can be further reduced by e(-) to form the hydroperoxide anion (HO2) under the reaction conditions, then O2(center dot-)and HO2- can be converted to O-1(2), finally the CN(- )can be oxidized by O-1(2) to CNO-. In addition, the recycle experiment and the characterization results of the g-C3N4-NS indicate that the g-C3N4-NS is stable.