We have already reported that the addition of guanidine phosphate increased the capacitance values as the electrode of an electrochemical capacitor for the carbon material derived from starch. If this method, that is, the addition of a compound containing hetero atoms such as N and P before the heat treatment for the carbonization is effective for improving the performance as the electrode of an electrochemical capacitor, then a high performance material can be developed by this method. Therefore, in this study, the effect of the addition of guanidine phosphate with starch is investigated in detail. The change in the surface area, which could be caused by P, is the main reason for enhancement of the capacitance value. The wettability against water is not improved by the addition of guanidine phosphate. The electrical resistance decreased by the addition of guanidine phosphate. This decrease should be one of the reasons for the improved capacitance value. The electrical double layer capacitance per surface area did not increase, whereas the change in the pseudo capacitance could be one of the reasons for the improved capacitance value. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.