Objective:This research aims to investigate the impact of relations between the supplier and the customer on capital structure’s reversibility and also considers the moderating role of the fundamental uncertainty.

Methods: The current research uses a non-experimental, inductive, descriptive and correlation methodology. The data are extracted from the statistical sample which consists of 131 firms listed on Tehran’ stock exchange between 2011-2020. For testing research’s hypothesis, multiple variable regression pattern based on aggregated data is used.

Results: The results collected by assessing the research’s model show that relations between the supplier and the customer have a positive and significant impact on the capital structure’s reversibility. In addition according to the results of the study, when the cash volatilizes of firms are higher, the impact of relations between the supplier and the customer on the capital structure’s reversibility is decreased.

Conclusion : According to the results of current research, when the relation between the supplier and its major customers is at a high level, and the major customers are involved in firm’s strategic decisions, the capital structure’s reversibility is enhanced. Meanwhile, these relations increase the speed of capital structure’s reversibility until a certain level. According to results of this study, it is because the focus on customer and relations with the firm’s related parties have indirect relations with the reversibility of capital structure. And the fundamental uncertainty as the moderating variable of this research, weakens such a relationship.