Abstract: An illumination apparatus having adjustable color temperature and a method for adjusting the color temperature. The apparatus and the method can adjust the color temperature for different purposes. The illumination apparatus comprises: a blue-light source; a transparent filter lens set on the top of the blue-light source; a fluorescence layer of cerium-doped yttrium aluminum garnet (YAG) phosphor is spread onto the transparent filter lens. Replacing the yttrium and the aluminum in the cerium-doped yttrium aluminum garnet phosphor with gadolinium (Gd) and gallium (Ga) respectively can produce yellow lights of different wavelengths. Different yellow lights can then be mixed with the blue light to produce white lights of different color temperature.