A capacitor having a dielectric layer including a composite oxide, the composite oxide including a transition metal and including a lanthanide group element, a memory device including the same and a method of manufacturing the capacitor are provided. The transition metal may be titanium and the composite oxide may be nitrided. The method may include providing a precursor of a transition metal, providing a precursor of a lanthanide group element, and forming a composite oxide on the lower electrode by oxidizing both the precursor of the transition metal and the precursor of the lanthanide group element, the composite oxide containing the transition metal and the lanthanide group element.