A luminosity control device includes a photosensitive member to control the duty cycle of a square wave pulse oscillator. Then a high power switching tube and a high power gain tube are used to proceed with the luminosity control. Thus, when two cars meet, the luminosity of headlamps will automatically weaken so as not to dazzle the driver's eyes in the opposite direction. Should no car come in the opposite direction, the headlamps will automatically intensify the luminosity thereof. In this manner, it is possible to effectively control the luminosity of headlamps and thereby achieve the object of traffic safety.