To analyze risks, the expert has to refer to several sources of knowledge (scenarii of accidents, studies of the dangers, dangerous elements, damages, etc.). Nevertheless, the expert faces difficulties to find the useful information and use previous experiences in the already existing studies, because this knowledge is not well formalized and the applied terminology collides with several difficulties: incompletion, incoherence, contradiction…. To solve this problem and avoid this ambiguity, our study aims at elaborating an ontology of risks analysis to formalize better and to structure the knowledge in risks analysis. Indeed, an ontology aims at specifying explicitly the concepts relative to a given domain. This article presents our works state of progress and shows the importance and the contribution of this new concept to improve and rationalize safety knowledge and more particularly in risks analyses.