Abstract: The valley-filling service market has been carried out in China. Combined cycle gas turbine units have flexible power generation capability, meanwhile their generation cost is high. Therefore when the units is involved in the valley-filling service market, the economic benefits of such service must be carefully evaluated. A method is proposed to evaluate the economic benefit of combined cycle gas turbine units participating in the valley-filling market. A production simulation model is utilized to simulate the power system operation and forecast the generation of the units for the operating day. A cost model of combined cycle gas turbine units is proposed and economic benefit of participation of valley-filling service market is predicted by calculating the revenues of the units. The economic benefits of the units in the valley-filling service market are then predicted for realize the market participation decisions. Finally, case study is performed based on the analysis of the valley-filling service market using an actual power system.