This is in vivo study to evaluate the possibility of using the Root ZX® apex locator as a sole method of determining and verifying tooth length during root canal treatment. Patients scheduled for orthodontic extraction will have one premolar receive root canal treatment with only a pre-operative and final radiographs and using Root ZX® apex locator to determine the working length. The contra-lateral premolar will be treated following routine root canal treatment procedures with multiple radiographs (pre-operative, working length, master apical file, master gutta-percha, and final radiograph). Then, the teeth were extracted at the end of the visit and the accuracy of root canal filling length was evaluated. The study shows the electronic method was satisfactory in 63.7% of cases, versus in radiographic method is 45.4%. We conclude that none of the techniques was totally satisfactory in establishing the true working length.