An image processing unit including an always on circuit block and a non-always on circuit block is provided. When operating under a first operation mode, the non-always on circuit block receives a bias voltage from a power supply unit, so as to perform an image processing operation on an image input signal. When operating under a second operation mode, the non-always on circuit block stops receiving the bias voltage from the power supply unit, so as to stop the image processing operation, and at least a microcontroller of the non-always on circuit block is powered down. One of the always on circuit block and the non-always on circuit block controls the power supply unit to stop supplying the bias voltage to the non-always on circuit block according an event trigger signal, such that the non-always on circuit block enters the second operation mode from the first operation mode.