As Internet of Things (IoT) has developed, the number of connected entities has increased, allowing systems to interact with users and their environments in smart ways. For example, presence and environmental sensors enable the system to be aware of the user's state and environment, allowing it to provide helpful information for users. The connected entities include not only IoT devices for data acquisition and actuation but also interaction modalities for communication between users and systems. Multimodal interaction (MI) and augmented reality (AR) are enablers for enhanced user experience (UX) for IoT services. However, combining an IoT-enabled system with MI and AR without careful consideration may hinder the benefits of these technologies. Thus, understanding the technologies and target user group's characteristics depending on the application's context is essential. Design principles allow people—who are interested in UX and user interface (UI) development of IoT-enabled mobile AR applications—to gain knowledge about UX/UI design that considers both the technology and user aspects.This thesis aims to identify, propose, and validate design principles for mobile applications within IoT-enabled smart city domains, especially healthcare and energy management services. We identified the requirements and needs of MI and AR through a systematic literature review. We also found that studies of design principles for IoT-enabled mobile AR applications are limited. We designed, developed, and analyzed three IoT-enabled mobile AR applications along with their UX. From the state-of-the-art research, we compiled and categorized 26 existing design principles into seven categories. We derived and evaluated five new design principles based on the analysis of our developed applications. As a practical realization of the identified design principles, we provided examples of design principles through user interface mockups, which represent the re-designed interfaces of the applications. We expect that our findings will give insight into the UX/UI design of IoT-enabled mobile AR applications for researchers, educators, and practitioners interested in UX/UI development.