Interharmonics， whose frequencies are not integral number of the fundamental frequency， will lead to the rms value and the peak value fluctuation in the voltage waveform which in turn leads to flicker. The quantitative relationship between interharmonics and flicker is analyzed， and the total effect induced by a number of interharmonics is discussed. By studying the IEC flicker measurement principle， the deficiencies in its use for the interharmonics-fliker measurement is pointed out， then a method based on the frequency domain algorithm is proposed to evaluate the effect of interharmonics flicker. The algorithm uses spectrum analysis first to obtain the various interharmonics components parameters， and then use these parameters to calculate the total flicker effect， which can simultaneously assess the rms fluctuation and peak fluctuations. At the same time， the concept of interharmonics power flicker is proposed to analyze different contribution from different interharmonics， so as to identify the flicker source and provide the theoretical basis for the assessment of flicker accountability and punishment. Compared with the IEC flickermeter， this algorithm can effectively measure interharmonic flicker problem， and correctly identify the flicker source.