A semiconductor device includes a semiconductor substrate, a gate structure, a source region, a drain region, a first oxide layer, a field plate, and a second oxide layer. The gate structure is disposed on the semiconductor substrate. The source region and the drain region are disposed in the semiconductor substrate and located at two opposite sides of the gate structure respectively. The first oxide layer includes a first portion disposed between the gate structure and the semiconductor substrate and a second portion disposed between the gate structure and the drain region. The field plate is partly disposed above the gate structure and partly disposed above the second portion of the first oxide layer. The second oxide layer includes a first portion disposed between the field plate and the gate structure and a second portion disposed between the field plate and the second portion of the first oxide layer.