Website usability has been reported to have crucial effects on online purchases, but few theoretical models thoroughly examine its effects. In this article we adopt music composition theory to propose a theoretical model of website usability, assuming that composing good music is similar to designing a usable website. The relevance of nomological networks between three constructs of music (melody, harmony, and rhythm), seven usability constructs (consistency, content relevance, interactivity, learnability, navigability, readability, uniqueness), cognitive and affective appraisals, and online purchases are hypothesized. A field study was conducted to validate the psychometric properties of measurement items and the nomological networks for the proposed model. The study results demonstrate that the proposed model successfully explains a large variance of the effects of website usability on online purchases and can thus be considered as an alternative theoretical model of website usability.