In a method of manufacturing a dielectric structure, after a tunnel oxide layer pattern is formed on a substrate, a floating gate is formed on the tunnel oxide layer. After a first dielectric layer pattern including a metal silicon oxide and a second dielectric layer pattern including a metal silicon oxynitride are formed, a control gate is formed on the dielectric structure. Since the dielectric structure includes at least one metal silicon oxide layer and at least one metal silicon oxynitride layer, the dielectric structure may have a high dielectric constant and a good thermal resistance. A non-volatile semiconductor memory device including the dielectric structure may have good electrical characteristics such as a large capacitance and a low leakage current.