Background: The aim of the present study was to investigate the quality of root canal obturation performed by 6 th year undergraduate dental students at the Dental School, University of Mashhad, Iran. Materials and Methods: A random 200 sample records of patients who received endodontic treatment at the Dental School, University of Mashhad between 2009 and 2010 was investigated. The quality of root canal filling was determined in relation to the adequate density and length of root filling. Statistical analysis of the data was carried out using statistical package for the social sciences (SPSS 12.0, SPSS Inc., Chicago, IL, USA) with Chi-square test. P < 0.05 was considered as significant. Results: About 38% of teeth fulfilled the criteria of an acceptable root canal filling. Adequate length and density of root filling was found in 73% and 66% of teeth respectively. There was significant difference between maxillary and mandibular teeth according to the quality (P = 0.009), length (P = 0.039) and density (P = 0.005) of root filling. The frequency of root canals with an "acceptable" filling was significantly greater in the anterior teeth than in molars. Conclusion: The technical quality of root canal treatment performed by undergraduate dental students was found to be less than ideal. Thus, the training course of the students at the preclinic and clinic has to be revised.