Paying attention to mental health, particularly girls' mental health, is very important. According to religious teachings, happiness, as well as obedience to God, avoidance of sin, revival of values, and spiritual change, are among the most important factors of improving mental health. This descriptive research was conducted with the purpose of analyzing the relationship of mental health and happiness with demographic characteristics of girl students of University of Qom. The population included all girl students of University of Qom in the academic year 2017-2018 and 351 girls were selected as the sample by the stratified sampling. The data were gathered by Goldberg's general health questionnaire and Oxford happiness inventory and they were analyzed by the Pearson correlation coefficient, t-test, and analysis of variance. The results showed that there is a positive and meaningful relationship between mental health and happiness (r=0/526, P<0/05). In addition, happiness has a significant relationship with the aspects of mental health including physical health, anxiety and insomnia, social dysfunction, and depression, respectively, with correlation coefficients of 0/150, 0/336, 0/619, and 0/459 (P<0/05). There is a meaningful difference among students' happiness in terms of the variables of age, education, and marital status, but there is not a meaningful difference among students' happiness in different faculties (P>0/05). Mental health does not have a significant relationship with age and different faculties (P>0/05), but it has a significant relationship with education and marital status (P<0/05). According to the results, it is suggested that students are provided with the opportunity to increase their intrinsic and rational happiness which is also approved by religious teachings.