A method for controlling backlight of portable electronic device in hysteresis manner pre-sets the portable electronic device in a default mode and senses a background brightness. The portable electronic device is operated at a daylight mode when the sensed background brightness is larger than a first threshold. When the portable electronic device is operated at the daylight mode and the sensed background brightness is smaller than a second threshold, the portable electronic device is again operated at the default mode, where the second threshold is much smaller then the first threshold to provide hysteresis effect. Therefore, the backlight can be turned off when the background brightness is sufficient. A rapid switch between default mode and daylight mode can be prevented by the hysteresis-manner control and user dizziness caused by rapid modes switch can also be prevented.