Abstract : In this paper, we investigate the problem of decomposing an ontology in Description Logics (DLs) based on graph partitioning algorithms. Also, we focus on syntax features of axioms in a given ontology. Our approach aims at decomposing the ontology into many sub ontologies that are as distinct as possible. We analyze the algorithms and exploit parameters of partitioning that influence the efficiency of computation and reasoning. These parameters are the number of concepts and roles shared by a pair of sub-ontologies, the size (the number of axioms) of each sub-ontology, and the topology of decomposition. We provide two concrete approaches for automatically decomposing the ontology, one is called minimal separator based partitioning, and the other is eigenvectors and eigenvalues based segmenting. We also tested on some parts of used TBoxes in the systems FaCT, Vedaall, tambis, ... and propose estimated results.