A semiconductor device and a method of fabricating the same are provided. First, a first oxide layer and a nitride layer are formed on a base having a first region and a second region. Next, the nitride layer is oxidized. A part of nitride in the nitride layer moves to the first oxide layer and the base. An upper portion of the nitride layer is converted to an upper oxide layer. Then, the upper oxide layer, the nitride layer and the first oxide layer in the second region are removed. Thereon, a second oxide layer is grown on the base in the second region. Nitride in the second region moves to the second oxide layer.