A high electron mobility transistor (HEMT) device and a manufacturing method thereof are provided. The HEMT device includes a channel layer, a barrier layer, a first gate electrode, a first drain electrode and a first source electrode. The channel layer is disposed on a substrate. A surface of a portion of the channel layer within a first region of the HEMT device includes a polar plane and a non-polar plane. The barrier layer is conformally disposed on the channel layer. The first gate electrode is disposed on the barrier layer, and located within the first region. The first drain electrode and the first source electrode are disposed within the first region, and located at opposite sides of the first gate electrode.