Adaptive Hypermedia Systems (AHS) can be developed to accommodate a variety of individual differences, including learning style and cognitive style. The current research is an attempt to examine some of the critical variables, which may be important in the design of an Adaptive Educational System (AES) based on student's cognitive style. Moreover, this paper describes the design issues that were considered for the development of the system that are reported in the relevant literature. Throughout the development of the system, formative evaluation was an integral part of the design methodology. The results of the formative evaluation were used to improve our system in order to make the instruction more effective and efficient. Furthermore, the recommendations resulted from the formative evaluation could be seen as some points worth considering from designers of AHS.