A high electron mobility transistor includes a semiconductor channel layer and a semiconductor barrier layer disposed on a substrate. A source electrode, a gate electrode and a drain electrode are disposed on the semiconductor channel layer. A patterned dielectric layer is disposed on the semiconductor barrier layer, and between the gate electrode and the drain electrode. A first field plate is extended continuously from a side of the patterned dielectric layer to the top surface thereof, and has a step in height. A first dielectric layer is disposed between the semiconductor barrier layer and the patterned dielectric layer. A second dielectric layer covers the patterned dielectric layer. The dielectric constant of the patterned dielectric layer is higher than that of the first dielectric layer and the second dielectric layer.