Guanidine has been known as an important building blocks for pharmaceutical and chemical industry, due to their broad spectrum of bioactivities. However, the synthesis of guanidine requires stoichiometric amounts of desulfurizing agents leading to toxic extra waste generation. Herein, we reported the first catalytic guanylation process which can be performed at room temperature in a mixture of water and ethanol as a solvent using Ru(bpy)3Cl2 as a photocatalyst under visible light irradiation. The key benefits of this reaction include the use of catalytic process with low toxic base/solvent at ambient temperature.