High-voltage MOS transistors with a floated drain-side auxiliary gate are provided. The high-voltage MOS transistors include a source region and a drain region provided in a semiconductor substrate. A main gate electrode is disposed over the semiconductor substrate between the drain region and the source region. A lower drain-side auxiliary gate and an upper drain-side auxiliary gate are sequentially stacked over the semiconductor substrate between the main gate electrode and the drain region. The lower drain-side auxiliary gate is electrically insulated from the semiconductor substrate, the main gate electrode and the upper drain-side auxiliary gate. Methods of fabricating the high-voltage MOS transistors are also provided.