A high electron mobility transistor (HEMT) device including a substrate, a channel layer, a barrier layer, a p-type gallium nitride (GaN) spacer, a gate electrode, a source electrode, and a drain electrode is provided. The channel layer is disposed on the substrate. The barrier layer is disposed on the channel layer and has a protruding portion. The P-type GaN spacer is disposed on a side wall of the protruding portion. The gate electrode is disposed on the protruding portion and the P-type GaN spacer. The source electrode and the drain electrode are disposed on two sides of the gate electrode.