Introduction: Life skills such as assertiveness may improve psycho-social ability and consequently can improve self-esteem. The aim of this study was to determine the effect of assertiveness training on first-grade schoolgirls' self-esteem in Tehran. Methods: This quasi-experimental study was conducted on 80 first-grade schoolgirls in Tehran who was selected by multistage cluster sampling method. First Cooper Smith’s self-esteem Inventory (SEI) was fulfilled by 410 qualified students. Then students, who obtain less than moderate in SEI, divided randomly into two control and intervention groups. For intervention group, we carried out a two-hour group training program for eight sessions. Two weeks later both intervention and control group fulfilled SEI for comparing. Gathered data was analyzed in SPSS v.16 software by using descriptive and analytic statistic. P-value<0.05 was considered significant. Results: Most of students were living in two-parent families (93%) and only 5% of them were the only child in family. Also finding showed there was significant difference between self-esteem of students in intervention and control group after assertiveness training. Before training intervention, the independent t-test indicated that there was no difference between the experimental and control groups in term of the amount of self-esteem in the students, but this amount increased in the experimental group after the intervention in comparison to before the intervention from 27.57±3.08 to 34.23±5.91 (p<0.05). In addition, there was difference between dimension of self-esteem, the highest rank referred to educational dimension and the lowest rank referred to social dimension. Conclusion: According to the results, it seems more counseling services and life skill training in workshop especially assertiveness training, is required to improve students' self-esteem. It may lead to increase knowledge and improve skill and therefore, improve mental health.