The length of root canal has to be measured for endodontic treatment. Several electronic apex locators were developed to measure the length of root canal by other researchers. And their accuracies were verified by X-ray or micrometer method. But these methods did not consider the non-linear bends of pulp and had ${pm}0.5mm$ error which was large enough to measure the length of root canal. The purpose of this study is the introduction of a new method to measure the length of root canal and the verification of the accuracy of an electronic apex locator using a Micro-CT. The length of root canal of 6 teeth were measured with the electronic apex locator. When the electronic apex locator reads 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm length of the file which was inserted in the hole of the tooth to measure the length of root canal. The average (${pm}$Standard deviation) length of root canal of 6 teeth measured by the Micro-CT was $0.49{pm}0.03,;0.59{pm}0.04,;0.68{pm}0.03,;0.78{pm}0.03,;0.90{pm}0.04;and;1.01{pm}0.03mm$, respectively. The maximum error of the electronic apex locator was 0.06 mm.