This paper addresses a new concept for determining the loss quota of any transaction and any participant entity in a multilateral open access power system. This new concept, which is based on the incremental transmission loss allocation (ITLA) technique, is proposed for application on multilateral open access power systems. This method considers trader entities that have no analogy in the electrical network and classic ITLA for which load flow study could not be applied. Using Optimal Power Flow (OPF) in the proposed method overcomes the problem of dependency of the final decision to the slack bus selection, which is the worst disadvantage of the ITLA method. The proposed approach is tested on IEEE RTS 24-bus network with satisfactory results.