An illuminating apparatus and control method is adapted to keep a constant total brightness of the ambient light and light produced by the illuminating apparatus. The illuminating apparatus has a control module storing an objective brightness and detect the ambient brightness through an optical sensor. The control module turns on a lamp according to a comparison result of the objective brightness and the ambient brightness. Hence, a total brightness of the lamp's brightness and the ambient brightness is adjustable to be equally bright as the objective brightness. The illuminating apparatus of the invention does not provide excessive brightness and accordingly conserves electric power energy.