Background: Infidelity is an interpersonal traumatic incident that can have cognitive, emotional and behavioral consequences for the victim. This study aimed to explain the effectiveness of Robert Enright Forgiveness Inventory (EFI) on the marital quality of women affected by infidelity in Ahvaz. Methods: In this research, a single-case experimental design was used. The population included all women affected by infidelity referring to Ahvaz counseling centers in 2016. The study sample which included 4 women referring to Ahvaz Avaye Daroon Center was selected using available sampling method. The participants responded to the Marital Quality Questionnaire during the treatment and follow-up phases. The data were analyzed using visual analysis, clinical significance (Reliable Change Index and Normative Comparison), and the recovery percentage formula. Findings: The results indicated that the marital quality increased in women in the treatment (40.37) and (44.88) follow-up phases. According to the normative comparisons, t was bigger than the table with the degree of freedom of 32; therefore, it can be argued that there was no clinical difference between the normal and abused groups in terms of marital quality. Conclusion: Robert Enright Forgiveness Inventory (EFI) aims to change beliefs, advocate positive thinking, increase the sense of tranquility, reduce the sense of revenge and teach communication skills. These measures can help increase the marital quality of women affected by infidelity.