Background & Objectives: Because of the high importance of mental health in different sections of society and the pivotal role of psychology students, so promoting their mental health can improve mental health in society. Mindfulness is one of the factors affecting self-acceptance and positive interaction with others. Mindfulness-based interventions emphasize flexible attention, openness and curiosity. Psychological flexibility is the ability of individuals to focus on the current situation and use the opportunities of that situation to step in the direction of internal goals and values despite the presence of challenging or unwanted psychological events. agreeableness is one of the five main personality traits of each person that indicate the inner desire and tendency of the person to respect, coordinate, and adapt to others. Psychological flexibility and agreeableness are among the examples of mental health. Emotional intelligence, on the other hand, is one of the psychological characteristics of an individual. This study aimed to investigate the effectiveness of mindfulness on cognitive flexibility and agreeableness by moderating the role of emotional intelligence. Methods: This applied research was a quasi-experimental study with a pretest-posttest and a 3-month follow-up design, and a control group. The study population consisted of students of Psychology studying in the academic year of 2019-2020 at the Science and Research Branch Tehran Islamic Azad University, Tehran City, Iran. Data were collected using Bar-On Emotional Quotient Inventory (Bar-On, 1980) and 60 students with emotional quotient score less than 3 standard deviations below the mean were purposively selected and randomly assigned to the experimental and control groups (30 students in each group). The inclusion criteria were to be a student in the Master's level of 2018-2019 Islamic Azad University of Science and Research Branch Tehran in the field of Psychology, age range from 20 to 60 years, and willingness to participate in the research project. The students selected as samples participated in the research knowingly and with full consent. The exclusion criteria were the absence of more than two sessions from the training program, unwillingness to continue attending the training program, and not doing the homework and exercises of the training program for two consecutive weeks. In the pretest, posttest, and follow-up, both groups completed the Acceptance and Action Questionnaire-II (AAQ-II) (Bond et al., 2011) and the Subscale of agreeableness in NEO-Personality Inventory (NEO-FFI) (Costa & McCrae, 1985). Then, the experimental group received 8 sessions of mindfulness, whereas the control group did not receive any intervention. In the descriptive statistics section, central and dispersion indicators such as mean and standard deviation were calculated, and in the inferential statistics section, variance analysis with repeated measures and Bonferroni post hoc test were performed. To compare two groups in terms of gender, marital status, and age (demographic variables), the Chi-square test was used. All statistical analyses were conducted in SPSS version 22 software. The significance level of the tests was considered 0. 05. Results: The results showed that the variance analysis of psychological flexibility and agreeableness variables was significant for the intragroup factor (time) (p<0. 001) and intergroup factor (p<0. 001). In the experimental group, the mean scores of the variables of psychological flexibility (p<0. 001) and agreeableness (p<0. 001) in the posttest and follow-up phases were higher than in the pretest phase. Also, significant differences between the posttest and follow-up stages were observed in the mean scores of psychological flexibility variables (p=0. 035) and agreeableness (p=0. 011). The results indicated that the effectiveness of mindfulness training in the follow-up phase in the experimental group was not permanent for both variables and significantly decreased in the follow-up. Conclusions: According to the results, mindfulness training can effectively improve the psychological flexibility and agreeableness of students with low emotional intelligence.