The OWL is a language for representing ontologies but it is unable to capture the uncertainty about the concepts for a domain. To address the problem of representing uncertainty, we propose in this paper, the theoretical aspects of our tool BeliefOWL which is based on evidential approach. It focuses on translating an ontology into a directed evidential network by applying a set of structural translation rules. Once the network is constructed, belief masses will be assigned to the different nodes in order to propagate uncertainties later.