Abstract: In the operational modal analysis of rotating machines, the periodic excitations introduce harmonic modes into structural modes. Based on the different spectral kurtosis values between a structural mode and a harmonic mode, an approach for detecting harmonic modes is proposed. The definition of spectral kurtosis is introduced and an optimized fast algorithm of its estimation is presented. The frequency spatial domain decomposition modal identification method is applied to separate harmonic modes from structural modes. Finally, a simulation example and a real experiment are used to validate the approach, in which the harmonic modes are successfully distinguished and removed.