This study aimed to determine the structural relationships between attachment styles and internet addiction, regarding meditating the role of self-esteem. This study was descriptive-correlational. Participants were 300 undergraduate students of Tabriz University in 2018-19, selected by multistage cluster sampling. The instruments were the internet Addiction Questionnaire (Yang, 1998), Adults Attachment Inventory (Hazan and Shaver, 1987) and Self-esteem Scale (Rosenberg, 1979). Data were analyzed by using the structural equation modeling method and the results showed that the measured model has a relatively favorable fit with the theoretical model (RMSEA=0.04, CFI=0.91, GFI=0.92). Attachment styles, in addition to their direct role, could significantly explain the symptoms of internet addiction through self-esteem. Based on these findings, it can be concluded that deficits in parental functions and insecure relations with caregivers affect individuals' self-esteem, and fluctuating self-esteem is associated with problematic behaviors like extreme tendency to the internet. It is suggested to consider the role of attachment styles and self-esteem in prevention and treatment interventions of internet addiction.