In methods of forming an oxide layer and an oxynitride layer, a substrate is loaded into a reaction chamber having a first pressure and a first temperature. The oxide layer is formed on the substrate using a reaction gas while increasing a temperature of the reaction chamber from the first temperature to a second temperature under a second pressure. Additionally, the oxide layer is nitrified in the reaction chamber to form the oxynitride layer on the substrate. When the oxide layer and/or the oxynitride layer are formed on the substrate, minute patterns of a semiconductor device, for example a DRAM device, an SRAM device or an LOGIC device may be easily formed on the oxide layer or the oxynitride layer.