This paper applies the theory of Rational Conjectural Equilibrium to analyzing Duopoly Firms' welfare and effect under the circumstances of product differentiation, demand and marginal cost asymmetry We gain three relation t heorems of Duopoly Firms with its conditional parameter, and analyze the relation between Duopoly mode with Cournot model and society optimum model, and put forward that structure is vital factor in the theory research of SCP (structure conduct performance) Even if in the same structure, different c onduct could lead to different performance and welfare