SUMMARY
Assertiveness is important for teenagers, that allows teens to put themselves and perform the strategic, directed, controlled and steady activities, so teens can avoid the negative behavior. Assertiveness is consist of by several factors, one of which is influenced by self-esteem. A person with a positive self-esteem has a positive self-concept. This study aims to empirically examine the relationship of self-concept with assertiveness in class X Kesatrian 2 Semarang High School.The samples in this study using cluster random sampling technique and obtained a sample of 104 students. Data mining method using two scales of psychology. 36-item scale with a valid Assertiveness Scale (a = 0.922) and the Self-Concept Scale 36-item valid (a = 0.932). Data were analyzed using simple linear regression.The results showed a correlation coefficient (rxy) of 0.706 with p = 0.000 (p <0.05) which means that there is a significant positive relationship between self-concept and assertiveness. The higher self concept generate the higher assertiveness students, and conversely the lower self-concept generate the lower assertiveness in class X students of Kesatrian 2 Senior High School Semarang. Effective contribution to the self-concept of assertiveness in class X Kesatrian 2 Senior High School Semarang are 49.9% and the remaining 50.1% is explained by other factors.