Background: Mental health is one of the main aspects of social well-being. Tehran -capital of Iran- is metropolitan, where the mental health status of citizens is not prioritized effectively. Objectives: The purpose of this study was identifying contributors of mental health inequality between lower and higher economic groups in Tehran through Oaxaca- Blinder method. Methods: The study was conducted by the data of Tehran’s Urban Heart Survey- Round 2 (2012). Through a three- stage stratified and clustered sampling method, 34,700 were selected as samples. The mental health status was measured by the General Health Questionnaire 28- items (GHQ- 28) and the quantity of the inequality in mental health was measured by corrected concentration index. The Fairlie’s decomposition approach was performed in STATA 14. Results: The corrected concentration index were: -0.0967 and -0.1004 by Erreyger’s and Wagstaff’s approaches. Being of the Iranian origin, disability conditions, employment status and smoking were identified as the main contributors of inequality in mental health among lower and higher economic groups. Conclusion: Thus, re-organizing strategies and plans on promoting the socio- economic status of non-Iranian residents, improving employment opportunities, developing well-designed environment for disabled individuals and supporting plans to reduce smoking is recommended to the urban policy makers.