We present in this paper a model of semantic representation of a dynamic virtual environment. Our model is embodied into a simulation architecture of a virtual city. The objective is to enable agents to make action decision consistent with the semantic state of their environment. The idea is to represent the services offered by the environment into an ontology of services and deduce the available object that has the best quality of service at any given time of the simulation. For this, we defined a layer of the ontology containing the general concepts of representation of the environment. This representation allows us to apply a unified model of reasoning to infer elements of knowledge useful for decision-making agents.