Objects are considered as basic components of software system in object orie nted software construction.This paper presents an ODL(object description language)and the denotational semantics of its main constructs.The formal counterparts of objects are given by theory presentation in SOP calculus.Attributes(structure)and operations (behavior)are integrated in coherent logical units around which the notion of encapsula tion is formalized.Aggregation,inheritance etc.are formalized as specification constructs which allow people to assemble large specification from existed ones/extend agiven speci fication.Further,the notions of inheritance and subclass are clarified.