For a system operator (SO), the accurate arrangement of various levels reserve capacity and generator unit commitment is quite essential to decrease operational cost under the condition of maintaining a balance between supply and demand. However, the renewable energies sources (RES) are becoming increasingly popular recently. Their uncertainty and uncontrollability may complex the mentioned problem. In view of this, this paper proposes a two-stage generator unit scheduling approach for day-ahead electricity market operation to ensure compliance with grid codes while reducing operating costs. In the first stage, the fast Fourier transformation (FFT)-based analysis method is proposed to precisely estimate the requirement of various levels frequency-regulation reserve capacity (FRRC), which can not only avoid the extra cost caused by excessive reserve capacity, but also ensure the fluctuation resulted by the high RES penetration can be dealt with. Besides, the second stage is a joint optimization strategy which integrates economic dispatch (ED) and FRRC optimal allocation considering the grid security restrictions and the characteristics of the generator units, such as operational cost and ramping rate. All the computational results are verified by power flow analysis via PSSE software for ensure the feasibility. The proposed method is verified via the power system in Taiwan with various possible scenarios according to the future energy development planning.