A deep study on the information process activity is very important to design and develop informarion systems．In this paper，we will introduce a informatlon process model Information Activity Chain， by which we can study the difference and relationship of the key concepts，such as event，information and knowledge．The usage of these concepts is SO chaotic，that has stunted the development of information systern design theory．Then，we give a‘semi-formalization’description of the model based on object oriented methodology．Finally，we use the model to give a category of information systems．