and Objective: One of the main factors in root canal therapy is determination of the length of root canal. Radiography is currently the most useful method in determining root canal length. Nowadays, electronical equipment (Apex-finder), which determines the length of root canal, is produced and the objective of this study was to determine the efficiency of this equipment. Methods: In this study, 62 monocanal teeth of 45 patients who were referred to endodontics department, faculty of dentistry,

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... Babol medical sciences university was studied. At first, paralleling technique of periapical radiography was done on this tooth and the length was determined by electronical Apex-finder (Formatron V) and then obtained lengths with Apex-finder and paralleling technique of periapical radiography were separately recorded and finally a comparison was done between two methods. Findings: Statistical analysis of the results showed that in 30.6% of cases, the obtained lengths of Apex-finder and paralleling technique of periapical radiography were conforming and in 54.8% of cases, Apex-finder with acceptance of ±0.5mm error in comparison to paralleling technique of periapical radiography indicates acceptable lengths. Conclusion: This study showed that Apex-finder unit in determining the length of the root canal in comparison to paralleling technique of periapical radiography had a lower efficiency. In order to obtaining more reliance, paralleling technique of periapical radiography should be used with Apex-finder.