An adjusting device for head light assembly includes a motor fixed to a frame and the output shaft of the motor extends through a guiding passage in the frame and connected to a driving rod which includes outer threaded periphery. An activation rod has a spherical head at a first end thereof so as to be connected to an adjusting member of the light bulb and a hole with an inner threaded periphery is defined axially in a second end of the activation rod. The outer threaded periphery of the driving rod is rotatably engaged with the inner threaded periphery of the activation rod so that when the output shaft rotates, the driving rod is co-rotated so as to move the activation rod away from or close to the frame.