Knowledge sharing has become an imperative and challenging issue in the E-health field for improving total quality of services. However, since the E-health subject is a multi-disciplinary and cross-organizational area, to have shared concepts, vocabulary plus a specification of its intended meaning, namely an ontology, has been an obstacle to develop e-health system. We will in this paper propose a knowledge sharing ontology on the basis of Activity Theory. We believe that Activity Theory can provide high level and rich ontology for the developers of E-health system to encompass the multi-disciplinary and cross-organizational knowledge. We validate our approach in the end by demonstrating our project IMIS (Integrated Mobile Information System for Healthcare) that uses the activity theory as ontological architecture for the construction of the whole system.