Holonic manufacturing systems have been recently presented to meet the various needs of customer rapidly. Holonic manufacturing systems consist of several holons which may perform decision making for the achievement of their objectives. Holonic manufacturing systems are characterized by cooperation and negotiation among holons. The problem of conflict can be occurred during communication between holons. Then the negotiation is required for the holons. This paper deals with a mathematical model for the mediation of negotiation between holons in the holonic manufacturing systems. In this paper an evaluation function for mediation is modeled using the utility function.