Background: Emotional maturity and mental health are the foundation of a successful marriage and indeed emotional maturity is the key to a satisfying happy life. Objectives: This study aimed to assess emotional maturity and mental health among couples referred to pre-marriage health centers in the city of Karaj. Methods: A cross-sectional study was conducted in the main center of pre-marriage testing in Karaj from March 2012 to August 2012. Using randomized sampling method 547 couples participated and filled the questionnaires, including a demographic scale, emotional maturity scale (EMS), and Mental Health Inventory (GHQ 28). Data were analyzed using ANOVA, t student, Pearson, and Multiple Regression tests to assess the correlation between variables. Results: Results showed a significant positive relationship between emotional maturity and mental health of couples (r = 0.67; P < 0.05). There was an association between couples' emotional maturity and sex, religion, occupation, and previous marital history. Also, the relationship between mental health of couples and gender, place of birth, employment history and previous marital history was significant. Regression analysis also indicated that mental health and having a permanent job were predictors of emotional maturity (P < 0.05). The mean score of emotional maturity was detected 91.3 ± 21.6, which was interpreted as emotional instability, and the mean score of mental health was determined 16.2 ± 9.9. Conclusions: The mental health and having a constant job can predict the level of emotional maturity. In addition, the results suggested that emotional maturity has a positive correlation with mental health.