The Thai power industry has a partial liberalization structure, which encourages a partial competition under supervision of the Thai Department of Energy. To this end, the author proposes a mathematical model with equilibrium constraints (MPECs) whose upper-level problem depicts a stated-owned utility, the Electricity Generation Authority of Thailand (EGAT), deciding on optimal power generation capacities and generation quantities given prices and interaction of other power producers in the lower level problem. The MPECs are reformulated as mixed-integer nonlinear program (MINLP) using Special Order Set 1 (SOS1) transformations and linearizations. The main contributions in this paper are two-fold. This paper develops a novel, mathematical program with equilibrium constraints to simulate the Thai power market. Second, the paper analyzes the complicated structure of power market in Thailand.