Introduction: Working length determination is important in successful endodontic treatment and retreatment. This study evaluated the accuracy of two electronic apex locators Root ZX and RaypexÒ6 (EALs) in determining the electronic working length (EWL) of the root canals in endodontic treatment and retreatment.Materials & Methods: Access cavities were prepared on forty extracted, single-rooted human teeth and the actual working length (AWL) of the canals was determined. In the first phase of the study, primary EWL of un-instrumented teeth was measured and compared between two EALs. In phase II, all of the teeth were pre-flared and divided into the control (n=10) and the retreatment groups (n=30). Canals in the retreatment group were obturated by the lateral condensation technique using Gutta percha and sealer. After 15 days, gutta-percha was removed, and then the secondary EWL was recorded and compared between the two devices, in treatment and retreatment groups. Data were analysed by paired t-test and t-test.Results: Significant differences were found between both EALs in treatment and retreatment phases of the study (p<0.001). Both EALs showed increased accuracy in retreatment group (p<0.001). However, no statistically significant differences were found between the control and retreatment groups in the second phase of the study for Root ZX (p=0.929), and RaypexÒ6 (p=0.937).Conclusion: Accuracy of the two EALs was similar and acceptable. EWLs determined by Root ZX were closer to the AWL. The EWL determination after pre-flaring improved the accuracy of EALs and root canal obturation remnant materials did not have any clear effect on the accuracy of these EALs.