A handheld electronic device is provided. The handheld electronic device comprises a main body, a processing unit, an image projection module, a motion sensor, a first light sensor and a second light sensor. The processing module is disposed in the main body to generate display data. The image projection module generates a projection image according to the display data. The motion sensor senses an orientation of the handheld electronic device. The first and second light sensors sense an ambient light. The processing unit generates the display data according to the ambient light sensed by the first light sensor when the processing module determines that the orientation is a first orientation and according to the ambient light sensed by the second light sensor when the processing module determines that the orientation is a second orientation.