IT Capabilities are vital to the competitive performance of firms in contemporary business environment. Firms can no long have competitive advantages by obtaining affordable information technology resources. Rather organizational IT capabilities are often cited as one of the most important factors that influence firm performance. The purpose of this paper is to enhance the understanding of IT capabilities and find out how the differences in IT capabilities explain the variation of firm performance. By using concepts from resource-based view of firms, this paper argues that IT capabilities influence firm performance through information system-business processes successes and enabling institutional forces and that firms should combine their technical capabilities, sourcing capabilities and implementation capabilities in such a way to generate synergy effect which can be described as assimilation capabilities. Therefore, opening the black box of IT capabilities has implications for both practitioners and researchers.