This paper presents Organization-Role-Entity-Aspect (OREA) model, an organizational model for multi-scale and multi-formalism description of complex systems. OREA is based on the assumption that one issue in complex system simulation is to integrate multi-scale and multi-formalism representation. To achieve this issue, we use an approach based on organization-centered multi-agents systems and Discrete Event System Specification (DEVS) formalism. While the organizational approach allows to deal

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... ith an explicit representation of global and local levels, DEVS formalism allows integration of models of different types to describe a system perceived at different scales. Integration of OREA formalism within DEVS allows multi-formalism specification of a model both at global and local levels. In addition, this allows specification of the social structure of a complex system following DEVS formalism. In OREA, the organizational structure is specified without any assumption on entities structure. The roles description in OREA concerns only the detailed description of interactions within organization. The way that individuals conceive their system and make decision is defined through the concepts of aspect. OREA is applied in implementation of a generic model for carbon dynamics simulation in West-African Savanna. Future works would concern the integration of organizational dynamics and holonic representation in OREA.