Introduction and aim: Social capital as a set of relationships based on trust does not exist within a person or a social structure, but takes shape within the space between people. The present study has been developed to examine the impact of social capital on the quality of life of drug addicts.Research Method: In this study we have utilized the correlational method with an applied purpose. The study population included all addicts referred to the Welfare - Treatment Centers of Tehran, 380 of whom have been selected as the sample population based on the accessible sampling method. We have collected data using two questionnaires on social capital and life quality both have formal validity and the reliability has been calculated 0.81 and 0.78 by the Cronbach's alpha coefficient.Findings: All hypotheses have been confirmed according to the findings of the study. Meaning that social capital weakness of drug addicts has reduced the quality of life of these individuals.Conclusion: The social capital in the present research has been divided in two cognitive and structural dimensions. The cognitive dimension deals with the intangible part such as values, beliefs, attitudes, behaviors and social norms; and the structural dimension of social capital involves the structures and nets comprising collective decision-making and mutual responsibility processes. People with a history of drug abuse in life tend to have little relationship with their relatives lacking a common language to exchange thoughts with and express feelings to others. Such people have lost the power of sharing ideas with others and gaining a common experience, therefore, they suffer from social isolation and their quality of life is severely reduced as well and in case of increasing the cognitive dimension of social capital, their quality of life will also be improved. There is a significant relation between the structural dimension of social capital and quality of life of drug addicts. Keeping relationship with drug abusers, on the one hand, and having no ties with healthy people, on the other hand, can be of the major factors pushing people towards drug addiction. Due to isolation in the community and lack of connection with other people, drug abusers will be severely limited in terms of the structural social capital; consequently, their quality of life will be greatly reduced as well.