The effect on amplitude of EXAFS from harmonic caused by high order diffraction of crystal monochromator is discussed. The general expression and calculated results are presented. It is shown that the harmornic decreases the amplitude of EXAFS and the amount of decrease is related to such factors as intensity ratio of harmonic to fundamental, thickness and absorption coefficient of specimen. K edge EXAFS spectra with and without harmonic for different thickness copper foils have been measured respectively, and the distortion of single shell EXAFS amplitudes have been extracted. The measured results are consistent with calculated ones. Consequently, a method to correct the harmonic effect on EXAFS amplitude is proposed.