The aim of this study was to compare the length between the mesio-buccal and mesio-lingual canal of the mandibular molars before and after early coronal flaring at the different measuring time using several electronic apex locators. Fifty mandibular molars with complete apical formation and patent foramens were selected. After establishing the initial working length of the buccal and lingual canal of the mesial root using a surgical microscope (Carl Zeiss Co. Germany) at 25X with #15 K-file tip just visible at the foramen, radiographs were taken for the working length. After measuring the length of mesio-buccal and mesio-lingual canal (control group), the electronic lengths were measured at different times using several electronic apex locators (experimental groups; I-Root ZX, II-Bingo, III-Propex, IV-Diagnostic). After early coronal flaring using the K3 file, the additional electronic lengths were measured using the same manner. The results were as follows: One canal has a correct working length for the mesial root of the mandibular molar, it can be used effectively for measuring the electronic working length of another canal when the files are superimposed or encountered at the apex. In addition, the accuracy of the electronic apex locators was increased as the measurement was accomplished after the early coronal flaring of the root canal and the measuring time was repeated.