The emerging energy crisis resulting from a high price of crude oil has drawn attention about renewable energy all over the world. Thailand is a developing country located at the heart of Southeast Asia, with about 66 million of population as of 2006. The electricity supply industry structure under the government of Thailand has been managed by the Electricity Generating Authority of Thailand, the Metropolitan Electricity Authority, and the Provincial Electricity Authority for almost 50 years. According to the 2004 Power Development Plan, the country would have generation reserve at a marginal of 15%. Due to the rapid economic growth and difficulty in building new centralized generating facilities, this has the potential to create a security issue of electricity energy shortage and may develop a severe system blackout. Renewable energy issues have been widely discussed and debated over the country. Although solar energy is the most prominent renewable energy source due to the appropriate geographic of Thailand, it is still considered as a low-density energy source which requires a large area of installation. Therefore, for the long-term investment, wind power seems to be a good candidate because it is quick to install, needs no fuel cost, and is environment friendly. Therefore, this paper investigates the impact of the penetration level of wind power generation to be integrated into the Thailand system. System stability has been observed in various system configuration changes due to different control strategies of wind farms. The economic analysis of wind-farm investment has been also considered.