In a method of automatically powering on and powering off a display device, the display device is switchable between a power on mode, where the display device is operable to show an image frame, and a power off mode, where the display device is unable to show an image frame. The method includes the steps of: (A) while the display device is in the power on mode, enabling the display device to detect whether there is an image signal input; and (B) in response to non-detection of the image signal input in step (A), enabling the display device to switch operation to the power off mode. The method dispenses with the need to provide a power switch button on the display device, and power consumption of the display device can be further lowered due to operation in the power off mode.