In this study, we examine issues related to e-payment security from the viewpoint of customers. This study proposes a conceptual model that delineates the determinants of consumers' observed security and observed trust, as well as the effects of observed security and observed trust on the use of e-payment systems. To test the model, structural equation modeling is employed to analyze data collected from 219 respondents in Iran. This research provides a theoretical foundation for academics and also practical guidelines for service providers in dealing with the security characteristic of e-payment systems.