GRAIXPERT is a hybrid expert system of the GRAI Methodology (used for enterprise modelling) based on different reasoning (Case-based reasoning (CBR), decomposition, transformation) containing rules, reference models and a case base. It enables to aid specialists of enterprise modelling during a study. This paper presents the concepts of this tool. We focus our attention on the different reasoning used for this tool. Other tools (GRAISUC and GRAIQUAL) are also being developed for completing the expert system The objective is to be efficient for the improvements of the performance (quality, cost and lead time) of the enterprise in terms of diagnosis and design. Finally we complete the presentation with an example in order to illustrate the concepts presented.