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The proportions of sapwood and heartwood of trees have significant impacts on various uses. Electric resistivity tomography (ERT) and corresponding electrical resistance (ER) value maps were examined in Japanese cedar (Cryptomeria japonica D. Don), Taiwania (Taiwania cryptomerioides Hayata), and Luanta fir (Cunninghamia konishii Hayata) trees. The position of the sapwood-heartwood demarcation was measured on incremental cores from living trees and the corresponding ER of the sapwood-heartwood boundary was acquired from the ER map. A positive significant relationship was found between the maximum ER plus minimum ER values (ERmax + ERmin) and ER of the sapwood-heartwood demarcation from the tomographic data. The position of the sapwood-heartwood demarcation was determined by corresponding ER, and the critical ER can be established by the ERmax + ERmin value of the tomographic data. The results from this study indicate that ERT technique can be used to determine the position of the sapwood-heartwood boundary and can serve as a methodology in undamaged living trees of Gymnosperm species.