The purpose of this study was to investigate the effectiveness of the "Philosophy for Children (P4C)" program training on assertiveness and interpersonal relationships in elementary school students in Tehran. The present study was a descriptive, semi-experimental, pretest-posttest with control group. The statistical population of this study consisted of all male students in the second year of elementary school studying in one of the public primary schools in Tehran in the academic year 2018-19. From this community, by multi-stage cluster sampling method, region 2 was first selected and one school was selected from among the boys' primary schools in region (2), which includes 84 male students in the 4^th (28), 5^th (26) and 6^th (30) grades. 30 of them were randomly divided into experimental (15) and control (15) groups. The experimental group received 8 sessions of philosophy program intervention training for children and the control group did not receive any intervention. For data collection, Gambril and Ritchie boldness or assertiveness questionnaire was used. The validity and reliability of this test were reported by Gambril and Rigi (1975) equal to 0.39 to 70 and 0.87, respectively, and Bahrami (1996) have calculated the validity coefficient of this test as 0.88. Also, the Teacher Interpersonal Relationships Assessment Scale developed by Viro Davin (1980) was used. Viro Davin concluded by analyzing the questionnaire questions that this questionnaire is a useful tool for measuring aspects of students' social behavior (r=0.87) The results showed that the training of the philosophy program for children has a significant effect on the improvement of interpersonal relationships and students' assertiveness (P <0.05). It can be concluded that the children's philosophy program can be used for an effective interventional method to increase assertiveness and improve interpersonal relationships.