An attempt to assess the influence of diffusion on localized impedance measurements by means of probe method is presented. The influence has been demonstrated and discussed by experiments and computer simulations. When the probe is positioned within the diffusion layer, the measured localized impedance spectra could be different from the conventional electrochemical impedance measurements due to the heterogeneous concentration distribution. The deviations can be de density-dependent when the variation of conductivity is the predominant reason of the influence. When the influence of diffusion is predominately due to the redox potential difference at the probe, the deviation can be less de-dependent. The influence at low frequency range is more serious than at high frequency range.