Energy dispersive X-ray fluorescence measurements were made for trace elements using synchrotron radiation from a dedicated electron storage ring at the Photon Factory in Japan. Monochromatization of the excitation beam by a crystal monochromator is effective in improving the signal to background ratio, while the synchrotron continuum excitation attains an absolute detectability of better than pg. Trace elements in a thin sample down to 60 ppb are detected by monochromatic excitation when there is no line interference. The experimental geometry optimal for a high signal to background ratio is investigated experimentally. The practical advantages of monochromatic excitation for the selective excitation of a particular element are demonstrated; these include enhanced sensitivity, suppression of the effects of the matrix element, and removal of overlapping peaks. The effects of higher order reflections and the residual tail part of the diffraction curve are also discussed in connection with quantitative analysis.