Social commerce is defined as a new wave of e-commerce in which traditional e-commerce is mediated by social media and social networking services in order to promote online transactions and shopping-related information exchanges. One of the main application of social commerce is 'online brand communities'. Although utilization of social media in order to advance customer brand engagement has proliferated significantly in recent years, most of the companies do not meet the expected level of engagement of their customers. Overall, the key factors that could explain customer engagement are not yet thoroughly identified. Therefore, the current study develops a conceptual customer engagement model that aims to explain customer engagement in online brand communities. Customer engagement in brand social media is related to interactions and communications with the brand and other customers. In this study, the antecedents and outcomes of customer engagement dimensions will be investigated. As different dimensions of engagement can be influenced by factors from different aspects, it is important to cover as many categories as possible. Therefore, the model includes various factor categories including social, brand related, self-oriented and contend related factors drawn from relevant literature. The research instrument was designed, and a pilot study was conducted with 75 participants to assure content validity and to test reliability. The paper contributes to understanding the factors influencing each dimension of customer engagement to brand communities.