An object-based logic calculus is outlined in this paper, which is built on the trace calculus based on dynamic entities with local states. Compared with dynamic entities, objects have more good properties, such as encapsulation. To formalize the concept of encapsulation, a new concept of valid action is introduced. It is only through these valid actions declared by the very object that the state of the object can be accessed or modified. An axiomatization of the object calculus as an extension of the axiom system of the trace calculus is also contributed in this paper. Finally, the object semantics description and property deduction are shown.