This paper introduces a new model for inter-layer message communication called object mapping model that achieves separation between message preparation and destination selection. In this model, as opposed to traditional message delivery model, sender objects of messages need not indicate the destination objects of the messages. The messages can be intercepted by special objects called mapper objects. When mapper objects intercept messages, they can decide to which objects the messages should be sent. Using this model, layered cooperative systems can be realized by using objects to construct each layer and using mapper objects to realize object mapping relationship. And object mapping model has many advantages such as modularity, transparency, implicit invocation and dynamic mapping.