The present study aims to evaluate the quality of root canal fillings (RCFs) on extracted mandibular molars performed by undergraduate students during their preclinical training which spans 2 academic years. Endodontics records and periapical radiographs of 260 extracted human first and second mandibular molars without site specification consisted the sample of this retrospective cross-sectional study. All root canal fillings were performed by undergraduate students during the preclinical phase of their training The samples collected were assessed using radiographs based on three quality criteria: length, density, and taper. Chi-square test was used for statistical analysis and Cohen's Kappa coefficient was implemented to assess inter-rater reliability. The majority of the evaluated RCFs were of inadequate quality. Of the 260 mandibular molars included in the study, 67 (25.8%) were deemed sufficient in terms of overall technical quality across both academic years. 3rd-year students (44.5%) were significantly more efficient compared to their 2nd- year counterparts (12.0%) regarding the overall technical quality of RCFs (p < 0.05). RCFs performed by the 3rd -year students, were of relatively higher quality compared with those performed by the 2nd-year trainees. However, the overall technical quality of RCFs was inadequate. Improvement of the current curriculum should be considered by incorporating contemporary techniques and materials to enhance the performance of undergraduate students on RCFs, before entering the clinical phase of their training.