Determining the working length (WL) is crucial for the effectiveness of endodontic therapy, as the best results are achieved when the apical limit is correctly determined and established in close proximity to the apical constriction. The present study was conducted to assess effect of heat treatment of nickel–titanium instruments on the accuracy of an electronic apex locator integrated with endodontic motor. Materials & Methods: The study consisted of 25 extracted human mandibular incisors with fully developed apex of both genders. WL was determined visually using a clinical microscope and a manual stainless steel K-file #10. Instruments of Mtwo, Reciproc, Reciproc Blue, WaveOne Gold, Twisted File Adaptive, and Hyflex CM systems with diameters from 0.25 mm to 0.50 mm, were used to measure WL during root canal preparation. The electrical resistivity of instruments with a diameter of 0.25 mm from each system was assessed. Results: There was non- statistical difference in EAL accuracy was observed using instruments with different heat treatments and the same tip diameter of 0.25 mm (P > 0.05). There was no statistical difference in EAL accuracy was found when comparing instruments of the same system but with different tip diameters (P > 0.05). Conclusion: The EAL accuracy is independent of the tip diameter and the type of heat treatment applied to the NiTi alloy used in mechanized instruments.