Abstract: The specificity of power industry determines that the power generation market in electricity market belongs to an oligopoly market in which each generation company more or less possesses certain market power. On the basis of detailed analysis on the factors that influence the market power of generation companies, major evaluation indices for market power of generation companies in regional electricity market are discussed and established. By taking a transaction period in a regional electricity market of IEEE30-bus power system as an instance, this paper comprehensively evaluates market power levels of six generation companies by using of principal component analysis method based on economic dispatching. Results show that market power level of each company is different and that all evaluation indices affect market power level greatly. The works in this paper will provide certain basis and references for market regulators and operators to identify, evaluate and control market power of generation companies.