Objective: This study aims to investigate the effectiveness of transdiagnostic treatment on maladaptive personality traits and mentalized affectivity of patients with generalized anxiety disorder comorbid with depression. Methods: This was a quasi-experimental study with a single case method. The statistical population included people with generalized anxiety disorder comorbid with depression who were referred to counseling centers in Isfahan City, Iran in 2020. From this population, 5 people were selected via purposive sampling and they were intervened with transdiagnostic treatment through weekly sessions held individually. The subjects were assessed at the baseline, third, fifth, eighth, and tenth sessions, and during a 1-month follow-up period using the short version of the adult version of the diagnostic and statistical manual of mental disorders, fifth edition, personality questionnaire, and the mentalized affectivity scale. To report the data, we used visual analysis, reliable change index, improvement percentage, and statistical and clinical significance. Results: The visual analysis, percentage of improvement, and reliable change index (RCI≥1/96), showed that transdiagnostic treatment created statistically significant changes and improvement in mentalized affectivity components, negative affectivity, and detachment; however, it did not have a significant effect on other personality traits (antagonism, disinhibition, and psychoticism). Conclusion: Transdiagnostic treatment is an effective treatment to modulate the mentalized affectivity, negative affectivity, and detachment of people with generalized anxiety disorder comorbid with depression.