A power management method for a handheld electronic device is provided. In the present method, a G-sensor is used for detecting a normal vector of a plane of the handheld electronic device. The normal vector is then determined whether being directed toward a downward direction. When the normal vector is determined as being directed toward the downward. Accordingly, the handheld electronic device can be controlled to enter the power saving mode timely according to the positioned state of the device without going through complicated procedures such as operating a menu, so as to provide a more intuitive and convenient way for power management.