Mashups are new-generation of Web applications aimed at reutilizing contents and services provided by third-party components. In Web applications, usability is considered one of the most important quality factors. However, usability evaluation of mashups is usually relied upon their individual components. Therefore, there is a need for specific approaches in order to evaluate the usability of mashups by considering their specific characteristics as composite artifacts. This paper proposes a Usability Model for Mashups that can be used for evaluating the usability of mashups applications. This model breaks down the usability sub-characteristic from the ISO/IEC 25010 standard into other sub-characteristics and attributes. Finally, metrics are associated to these attributes to quantify and detect usability problems. In order to illustrate the feasibility of the approach, we present the evaluation of a mashup application using the proposed usability model.