The B notation is based on set theory, the language of generalised substitutions and first order logic. B specifications are composed of abstract machines whose states can be modified by operations. The rigour of B specifications is ensured by proof obligations which are automatically generated and proved by the B support tools. The mechanism of proof is based on mathematical reasoning and rules of inference. In this paper, we present a formal approach to specification and verification of an object-oriented system based on the B method. We lightly modify the structure of abstract machines to make it similar to classes in object-oriented notations. We introduce a simulation machine to simulate the execution and to check the correctness of a specified system. A case study is used to illustrate this approach