A nonvolatile memory device and a method of forming a nonvolatile memory device are provided. The nonvolatile memory device includes an active region of a semiconductor substrate defined by a device isolation layer, a tunnel insulating structure disposed on the active region, and a charge storage structure disposed on the tunnel insulating structure. The nonvolatile memory device also includes a gate interlayer dielectric layer disposed on the charge storage structure, and a control gate electrode disposed on the gate interlayer dielectric layer. The charge storage structure includes an upper charge storage structure and a lower charge storage structure, and the upper charge storage structure has a higher impurity concentration than the lower charge storage structure.