The Brazilian Electricity Sector has experienced strong structural changes, whose main objective is to create a competitiveness character to allow for the sector to grow not more for state investments, but now for the hands of the private capital. So, it is expected that supply can meet the growing demand in the last years, due to the lack of investments in the sector, and consequently reduce the risk of load deficit in the future. The Emergency Program of Thermal Plants, introduced by the federal government, will increase the offer of energy in the country with the construction of thermal plants, taking advantage of the availability of natural gas, and thus, will increase the participation of thermal generation in Brazil. That change will give more reliability to the Brazilian Electric System, reducing the risk of energy deficit due to the reduction of the level of water in the reservoirs of the hydro plants. Besides, thermal plants is a short-term alternative to Brazil, because its construction term is shorter than that of hydro plants, so this will increase the offer of energy during the transition for the competitive market, and so reducing the risk of load deficit in the future. Thermal Plants, in that new structure of the Brazilian Electric Sector, can be declared flexible either or inflexible. A flexible thermal plant is that where its energy can be negotiated in a WholeSale Energy Market, and so being subjects the volatility of the spot market, but it could obtain great profits. An inflexible thermal plant is that where its whole energy is negotiated by through bilateral contracts, that is, the energy generated is not negociated in the WholeSale Energy Market. This reduces uncertainties, but it also reduces the opportunities of great profits. The largest uncertainty of a project of the Brazilian electric sector is the price of electricity, and in countries where this sector was restructured, like in Brazil, it is determined through the short run marginal cost (SRMC), for energetic otimization model not published to the market. Thus, this dissertation intends to formulate a model for the price of electricity in Brazil, to evaluate a thermal plant using Real Options Theory and Monte Carlo simulation, and to compare the results with CASTRO´s dissertation: Evaluation of Capital Investment in Thermoelectric Generation Projects in the Brazilian Electricity Sector Using Real Options Theory.