To investigate the instantaneous structure of the local reaction zone of the HiCOT (High Temperature Air Combustion Technology) flames, the characteristics of ion current obtained from the non-premixed flames of propane with highly preheated low-oxygen air have been closely examined. Nonluminous (bluish-green) region is seen in the upstream part and luminous (bright yellow) region is seen in the downstream part of the non-premixed flames of propane with highly preheated low-oxygen air, while only the luminous region is seen in the non-premixed flames of propane with highly preheated regular air. The ion current fluctuations, obtained from the nonluminous region of the HiCOT flames, look quite similar with those obtained from turbulent premixed flames in the reaction sheet regime, implying that the unburnt mixture, burnt gas and local reaction zone exist separately. It has been clearly shown from the ion current that homogenous combustion does not occur but the local reaction zone exists separately from burnt and unburnt gases. The thickness of the local reaction zone in the nonluminous region of HiCOT flames is not broadened but is similar to that of the laminar premixed flames. Thickness of the local reaction zone in the non-premixed flames of propane with highly preheated regular air is estimated approximately twice as much as that in the nonluminous regions of the HiCOT flames.