The computation of object systems is an open and dynamic computing process with concurrent interactions, and the behavior of object systems presents the unification between internal causes and external causes, so that the research of dynamic behavior of object systems should not be separated from the discussion of static construction of object system. This paper aims at the specification of dynamic behaviors and interactions of objects, while a formal model for composite object systems is constructed to consolidate the formal abstraction and conception framework throughout the whole paper at first. A sorted CCS-like calculus named S/R is built up to clarify the computation process of object systems and to characterize the inherent properties of encapsulation, dynamic binding, interaction and concurrecy, etc. It is emphasized that the behavior of each object is restricted by its environment and the behavior of each component object should be consistent with the behavior of its composite object.