A possible mechanism for the field emission spectroscopy (FES) peak energy shift observed for lightly nitrogen (N)-doped chemical vapor deposition (CVD) diamond was investigated using natural IIb diamond as a reference. Combined ultraviolet photoelectron spectroscopy/FES spectra of natural IIb diamond indicated that the origin of field-emitted electrons is at the valence-band maximum and does not shift depending on the applied voltages. To further investigate the mechanism, FES peak energy was plotted versus emission current and the plot was best fitted to a straight line. The resistance of the diamond obtained from the slope was 10(9) Omega and almost 0 for natural IIb diamond and lightly N-doped CVD diamond, respectively. The result was confirmed to be consistent with the resistivity of lightly N-doped CVD diamond and natural IIb diamond. Therefore, the result strongly implies that the observed energy shift is due to the voltage drop at the field emission site due to the resistance of the diamond bulk. Details of a possible mechanism are explained. (C) 2008 American Vacuum Society.