In this paper the harmonic degrees between $alpha$ wave and $eta$ wave in visual evoked potential are analyzed by the bicoherence. The bicoherence analysis is based on an AR model which provides significantly better resolution than that of Fourier transform. The analysis results of visual evoked pope ntial are compared with the analysis results of background EEC. From the comparison results it is found that the harmonic degree of visual evoked potential is less than she harmonic degree of background EEG and the $eta$ wave of visual evoke potential unlike the background EEC contains the non harmonic property of a wave more than the harmonic properity