Abstract: Nonparametric estimation of power spectrum density (PSD) in vibration engineering, including the major approach as well as the periodogram method, is based on classical Fourier transformation (FT), but takes quite different forms. This paper discusses the influences of the FT forms on PSD estimation using non-parameter methods such as the periodogram method. First, several FT forms and PSD non-parameter methods are discussed. Then, it is shown that among different forms for PSD estimation only one has physical meaning consistent with the PSD concept. Finally, a general formula for FT and PSD estimation is proposed.