Introduction: The aim was to assess the quality of root canal filling and the frequency of procedural errors in endodontic treatment performed by undergraduate students at Riyadh Elm University, Riyadh, Saudi Arabia. Materials and Methods: Radiographic record of 306 postroot canals treated anterior and posterior teeth done by female undergraduate students between 2013 and 2019 were randomly selected. The root canal filling was evaluated using the following criteria: acceptable treatment comprises the following characteristics: uniform density, no voids, root canal filling 0–2 mm from the radiographic apex, and tapered: overfilling, underfilling, and inadequate density were considered unacceptable. Data were statistically analyzed. Results: A total of 116 root canal fillings were done in the anterior teeth and 190 in the posterior teeth. In which 69.9% of root canal fillings were acceptable, and 66% of the treated teeth were in the maxilla. Patients with abscess and periapical pathology in radiograph had 2.11 times higher odds of unacceptable root canal fillings compared to patients with normal periapical radiographs, and the association was significant with 95% (confidence interval: 1.27–3.50) after adjusting for age, number of canals, tooth position, and location. About 13.7% of the unacceptable root canal fillings were underfilled. Conclusion: The quality of root canal filling performed by female undergraduates was considered acceptable. Underfilling was the mostly identified treatment error. Hence, additional preparation is needed to improve the clinical aptitudes of the undergraduates for better endodontic treatment results.