The paper describes the application of MARKAL (MARKet Allocation) for comparing economic parameters of the future power plant Thailand between the combined cycle natural gas fired plants and nuclear power plants, which the first plant is planned to be built in 2020. All calculations for demand technologies in this study cover a time frame of 25 years from 2006 to 2030, which are grouped into 12 periods with each further divided into 8 time-divisions accounting for the 4 seasons and the daytime/nighttime. MARKAL simulation model have been chosen because of their adaptability to this specific task. The input data for calculation are cost elements, which are defined within the frame of set limits of expected occurrence given a probability distribution within the limits. The method is applied to determine competitive specific investment costs of power plants which are planned to be built in the future. The main candidates are combined cycle gas fired plants and nuclear power plants, which the first nuclear power plant is planned to be built in 2020 with the starting electricity production capacity of 1,800 MW then the production capacity is planned to be increased to 3,600 MW in 2021. All information about the plan for future power plants are cited from power developing plan 2007 (PDP 2007) by the Electricity Generating Authority of Thailand (EGAT). This paper summarizes the programming and calculating methodology, and presents selected results of cost analysis of the performance and costs between the types of power plants. Performed analysis shows that future competitive nuclear power plant is cost competitive with other forms of electricity generation except where there is direct access to low-cost fossil fuels. Comparing costs for nuclear power plants with other types of power plants, fuel cost only accounts for a minor portion of total generating cost; however, the capital costs for nuclear power plants are much greater than those for thermal plants. A discount rate of 10% is selected as most appropriate for analysis of the long-term technological choices. The Thai MARKAL energy systems model has been used to investigate long-term planning in the electricity generation options. A range of power sector specific parametric sensitivities have been performed to provide a systematic exploration of least-cost energy system configurations under integrated set of input assumptions. The results of the MARKAL model are used to develop selected scenario in future developments of reference energy system. The study also considers the potential of Clean Development Mechanism (CDM) in Thailand by comparing the avoid cost of CO 2 corresponding to nuclear power plant scenario. As a result, optimal scenario of generation expansion can be developed for Thailand electrical system. © 2009 SICE.