Electricity generation in Thailand is highly dependant on natural gas. Recent research has revealed that the Thai economy would become more vulnerable from high gas dependence in the power sector. This paper aims to assess the economic impact of gas dependence in power generation in the coming decades. To fulfil this objective, two scenarios of electricity capacity planning were developed and the results were analysed to understand the changes in gas dependence and the effects on import reliance. It is found that from 2011 to 2025, the average cost of natural gas for power generation will account for 2.41 % of gross domestic product (GDP) while high oil price in international energy markets would push this cost to 2.97% of GDP. In addition, reliance on fuel imports for power generation, particularly natural gas and coal, is going to be another crucial concern to the security of energy supply as the costs of these imports during the planning horizon will increase significantly at an average rate of 6.78% per year. (c) 2007 Elsevier Ltd. All rights reserved.