This study aims to establish a model-based approach for user interface design that simultaneously considers the system's information hierarchy, users' task procedure knowledge, and system interfaces. The approach is based on a framework that contains multiple interaction models to express both system elements and users' knowledge. The framework evaluates system interface through the interaction between user's knowledge on interface, task procedure and information structure perceived by the user in the system. The interface is evaluated by its contribution to the users' task performance and system navigation. These three factors were defined as design factors that affect users' task performance. Through the crosscheck process of models, the relation between information, interface, and task procedure is calculated into combined difficulty index (CDI) that expresses the difficulty of a system interface that users would experience during the use of system. A user test was conducted for the validation of the CDI. The difficulties of the interface of a mobile healthcare system were predicted with the CDI, and the predictions were compared with the experimental results, where the users' performance showed consistence with the prediction. (c) 2006 Elsevier B.V. All rights reserved.