Depression is a multifactorial disorder with various causes such as personality, cognition, family background, or interpersonal relationships and it is very unlikely each one of these factors be responsible for depression development by themselves (1).With the onset of adolescence, girls are reported to have depression at a rate two or three times higher than that in boys, with this sex difference remaining throughout their lives (2). Baumeister and Leary (3) believe that developing and continuing persistent interpersonal relationships is a fundamental human motivation. Also, research has consistently indicated that a low-quality of interpersonal relationships is associated with negative consequences on the mental health of young individuals (4,5). On the other hand, the positive qualities in parent-adolescent relations including high support, warmth and approval are correlated with lower levels of depression symptoms in adolescents (6,7,8). The importance of high-quality relationships with peers for young individuals' psychological well-being is a well-established issue (9). Conversely, studies investigating the way these positive and negative qualities are related to depression have not been considered. In a study, La Greca and Harrison (10) found that the presence of positive qualities in a relationship with a best friend protects the individual from social anxiety but not from depressive symptoms. They assumed that positive qualities such as support and self-disclosure are likely to cause the individual continue talking about the problem and focusing on negative feelings, leading to persistence of the depressive symptoms. They also demonstrated that a higher degree of negative quality significantly predicts social anxiety and depression. In the present study, we have assumed that the quality of relationships has a direct effect on depression.Loneliness is another variable that affects depression. Peplau et al. (11) defined loneliness as the gap between the desirable and existing levels in an individual's social relations. According to Perlman, loneliness results from the distance and gap between the individual's ideals and achievements in interpersonal relationships and intimacy; the greater this distance the more intense the loneliness (12). Different studies revealed that loneliness positively correlates with internalized disorders, such as depression (13,14,15). Boivin and Hymel (16) found that loneliness has a mediating effect on a child's social conditions and consequences of depression. Based on these mediating and probable effects of social conditions on the quality of interpersonal relationships, it can be argued that loneliness can play a mediating role in the relation between the quality of interpersonal relationships and subsequent depressive symptoms.