Mobile computing is a new computing paradigm. A main feature of this paradigm is dynamically changing the binding between com putational components and computational locations, which brings new requirements to the design of distributed systems. In this paper, an abstract model for mobi le component systems is proposed. Not only Location, Component, Mobility and Res ource Access as the basic elements of mobile systems are identified in the abstr act model, but modeling mobile systems by basic elements, relationships among el ements and relationship changes is also characterized. Based on set theory and o perational reduction rules, these elements, relationships and mobility mechanism s are specified in a formal manner. The analysis and formal specifications can b e applied to the design basis of mobile systems and their specification languages.