The use of computerized adaptive testing (CAT) has expanded rapidly over recent years mainly due to the advances in communication and information technology. Availability of advanced mobile technologies provides several benefits to e-learning by creating an additional channel of access with mobile devices such as PDAs and mobile phones. This paper describes the design issues that were considered for the development and the implementation of a CAT on mobile devices, the computerized adaptive test on mobile devices (CAT-MD). Throughout the development of the system, formative evaluation was an integral part of the design methodology. The recommendations, suggestions and the results of the formative evaluation were used to improve the system in order to make the assessment procedure more effective and efficient. These results of the formative evaluation are also presented here.