The invention relates to a user equipment of device-to-device communications and a resource selection method thereof. The user equipment includes a resource selection device and a transceiver device. The resource selection device is configured to create a state machine model, which includes an in-coverage state and an out-of-coverage state corresponding to a base station. The resource selection device is also configured to select a corresponding device-to-device communication resource in response to a current state of the user equipment during transferring of the user equipment from the in-coverage state to the out-of-coverage state or from the out-of-coverage state to the in-coverage state. The transceiver device is configured to perform a device-to-device communication with another user equipment according to the corresponding device-to-device communication resource. The resource selection method includes the steps corresponding to the operations of the user equipment.