A semiconductor device includes a substrate, having cell region and high-voltage region. A memory cell is on the substrate within the cell region. The memory cell includes a memory gate structure and a selection gate structure on the substrate. A first spacer is sandwiched between or respectively on sidewalls of the memory cell structure and the selection gate structure. First high-voltage transistor is on the substrate within the high-voltage region. A first composite gate structure of the first high-voltage transistor includes a first gate structure on the substrate, an insulating layer with a predetermined thickness on the substrate in a -like structure or an L-like structure at cross-section, and a second gate structure on the insulating layer along the -like structure or the L-like structure. The selection gate structure and the second gate structure are originated from a same preliminary conductive layer.