Despite its centrality to human thoughts and practices, aesthetics has largely been ignored in Web site design. Until recent years, studies begin to show that task-unrelated (aesthetic) qualities such as colour, graphics and music can play an important role in enhancing both usage and enjoyment of information systems as well as improving work quality. However, research progress has been greatly impeded by the lack of a conceptual framework for understanding aesthetics as well as empirically verified instrument to measure Web site aesthetics. Our research objective is to provide a sound theoretical framework of Web site aesthetics as well as a validated instrument to measure it. Based on extensive literature review on aesthetics in environmental psychology and Human-Computer Interaction (HCI), we propose a two-dimensional structure of Web site aesthetics concept in a cognitive-affective framework and develop a Web site aesthetics instrument in the context of electronic shopping. Initial item development, statistical analyses, and validity assessment (reliability, convergent validity, and discriminant validity) are described here in details. The contribution of this effort is twofold: First, it integrates theories from different domains to help us understand the concept of Web site aesthetics. Second, an instrument is provided for Web site aesthetics, which will facilitate future research in this field.