Mobile user interfaces are currently not well designed with considering mobility. The usability of mobile services becomes degraded while moving, since a user cannot pay enough attention to the service in a multi-task environment. Thus, it is important for mobile services to convey information to a user with less cognitive workload. In this paper, we address design issues to enhance the mobility of mobile services, and evaluate the workload caused from cognitive processes in a multi-task environment. Taking cross-modal and cross-device interactions into consideration, three factors are focused as variable parameters in the experiment: device type, contents format and information volume. Based on the experimental results, we discuss how these factors affect a cognitive process in a mobile interaction. Future work is also implicated in order to apply the findings to actual mobile user interface design.