This paper presents a bidding decision method for generation companies（GENCOS） based on evaluation of market effect power. First， the GENCOS are synthetically evaluated with two market effect power indexes. Then according to the differences of their market effect power， the bidding behaviors of GENCOS are analyzed with the objectives of max-profit， and the optimized bidding strategies including the price-taker strategy and price-affecter strategy are obtained respectively， with the decision processes for different types of strategies constructed as well. Simulations on a power market model show that the bidding strategies proposed can greatly increase the profit and hedge the risk of the GENCOS in the bidding competition.