A motorized sun screen for covering a vehicle window has a base unit and a screen unit which includes a lower horizontal rod mounted rotatably on the base unit, an upper horizontal shaft and a screen with a bottom edge connected to the horizontal rod and a top edge connected to the horizontal shaft. Each of a pair of pivot arms has a lower end portion mounted pivotally on the base unit and an upper end portion slidably engaging the horizontal shaft. An extension spring has two ends connected respectively to the lower end portion of the pivot arms. A drive unit is mounted on the base unit and includes a motor with a rotatable drive shaft which is coupled to one end of the horizontal rod. The motor is operable in a first mode, wherein the motor permits the extension spring to contract, thereby causing the pivot arms to pivot away from each other to move the horizontal shaft upwardly and stretch the screen. In a second mode, the motor causes the horizontal rod to rotate, thereby winding the screen around the horizontal rod. As the screen is being wound, the pivot arms pivot toward each other moving the horizontal shaft downwardly and the extension spring tenses.