This study presents a new method for calculating the optimal bidding strategies among Generating Companies (GENCOs) in the electricity markets with the assumptions of imperfect competition and complete information and with consideration of uncertainty in load forecast. The parameterized Supply Function Equilibrium (SFE) is employed for modeling the imperfect competition among GENCOs in which proportionate parameterization of the sole and the intercept is used. A pay-as-LMP pricing mechanism is assumed for settling the market and calculating the GENCOs’ profits. The fuzzy approach is utilized for modeling the uncertainty of load forecast and the result is compared with probabilistic approach. A nine GENCOs test system is used to show the efficiency of the proposed method.