Provided are a method of forming a dielectric and a method of fabricating a semiconductor device. The method includes forming a preliminary dielectric including Hf, O and an “A” element on an underlying layer. The preliminary dielectric is formed in an amorphous structure or a mixed structure of an amorphous structure and an “M” crystalline structure. The “A” element about 1 at % to about 5 at % of the total content of the “A” element and Hf in the preliminary dielectric. Through a nitridation process, nitrogen is added to the preliminary dielectric. The nitrogen-containing dielectric is changed into a dielectric having a “T” crystalline structure through a phase transition process, wherein the “T” crystalline structure is different from the “M” crystalline structure. An upper layer is formed on the “T” crystalline dielectric.