The study examined whether self-esteem lability mediates a structural relationship among anxiety attachment, avoidance attachment, and social anxiety. A total of 377 high schools students(193 males and 184 females) from Seoul and Gyeonggi-do participated in the study. The study conducted descriptive analysis and correlation analysis by SPSS 18.0, and structural equation modeling by Amos 26.0 to identify structural relationships. The results of this study were as follows. First, there were significant correlations among anxiety attachment, avoidance attachment, social anxiety, and self-esteem lability. Second, after controlling the general level of self-esteem, there was a mediating effect of self-esteem lability in the relationship of anxiety attachment and social anxiety. However, there was no mediation effect of self-esteem lability in the relationship between avoidance attachment and social anxiety. These results show that the self-esteem lability is an important mediating variable explaining the relationship of anxiety attachment and social anxiety even after controlling the level of self-esteem. The self-esteem lability is a key variable in understanding the social anxiety of high school students with anxiety attachment. It was further confirmed that avoidance attachment and anxiety attachment operates with different mechanisms. Implication of this study was discussed.