This paper presents a bi-level programming approach for determining optimal generation share to maximize the available transfer capability (ATC) considering Hopf bifurcation limit in the bilateral/multilateral transaction. In the deregulated electricity market of open transmission access, all the market participants try to utilize the transmission system fully; hence it is important to improve the oscillatory stability based ATC for allowing more electricity contracts in future without any transmission limit violation . In the multi-nodal case of bilateral electricity market and multilateral electricity market, ATC significantly varies dependent on the points of electric power injection in the transmission system. This paper suggests the method for establishing bilateral/multilateral contract based on the optimal generation sharing of injection power to maximize the ATC. Real coded genetic algorithm (RGA) is used as an optimization tool to solve the proposed approach. The proposed method is tested in WSCC 3 machine 9-bus system and New England 10 machine 39-bus system.