Aims: Toothache can deteriorate the quality of life. The purpose of this study was to assess the quality of life of patients with irreversible pulpitis before and after root canal therapy using the oral health impact profile (OHIP-14) questionnaire. Materials and Methods: This study was performed on 100 patients (64 females and 36 males) with painful irreversible pulpitis who seeking treatment. Data were collected using demographic andOHIP-14 questionnaires and a visual analog scale (VAS). Results: The mean OHIP score changed from 15.89±9.42 to 1.49±2.64, with improvement in the quality of life (p<0.0001). The mean pain score decreased from 5.61±2.87 to 0.17±0.53 postoperatively (P<0.0001). Before treatment, physical pain and psychological disability domains had the greatest adverse effect on the quality of life. Preoperatively, the pain related to mandibular teeth was higher than that of maxillary teeth (p<0.04). Males (β =0.466, P<0.0001) and lower education (β=1.3, P=0.026) show better change in OHIP.  The impact of root canal therapy on the level of pain was still significant after controlling for demographic and oral-dental variables (β=5.4, P<0.0001) Conclusion: root canal therapy significantly improves the quality of life in all related aspects by alleviating dental pain.