The worldwide mobile software market has grown dramatically since feature phones became popular in the early 1990s. In practice, mobile usability - which can be defined for a resource-constrained device in two ways, namely, User eXperience (UX) and User Interface (UI) - has been regarded as the key to gaining superiority in terms of both market share and customer loyalty. Unfortunately, de facto standards for software design and the development process, such as Unified Modeling Language (UML) and Rational Unified Process (RUP), do not seem to promote mobile usability in a systematic manner in practice. This paper proposes a systematic and generalizable approach to modeling and evaluating the properties of mobile usability, herein treating it as a first-class software quality from the perspective of software engineering. We devise a UX evaluation framework for mobile usability, which we call UX Evaluation Framework (UEF) throughout this paper. A UX is specified by inter-scene interactions between users and terminals of software products using Extended Menu Navigation Viewpoints (EMNVs); then, a model checker, NuSMV, is adopted to observe whether the EMNV model meets a set of given UX properties. Importantly, the analysis and design of RUP is extended to support the co-design of UX and UI so that major roles, activities and artifacts in the UX and UI can be explicitly monitored and controlled by stakeholders. Through case studies, we demonstrate that UEF works properly to treat software products that prioritize mobile usability. Consequently, UEF plays a key role in filling the gap between two research disciplines to address usability: software engineering and human-computer interactions.