Greenhouse gases reduction from the usage of solar hot water system aims to utilize the solar energy in the thermal process to substitute the fossil fuels. The effect of the system reduces the greenhouse gases such as Carbon Dioxide (CO2) released to the atmosphere and help relieving Global Warming problem. The problem has been taken into Thailand consideration and signed in Kyoto Protocal. Moreover, the system can promote the policy and the target of government in order to increase the renewable energy to be 8% within 2011. This research collected the data of solar hot water system from companies and distributors, government and university including data from field surveying with existing installation solar hot water system such as hospital and hotel. The research assessed the energy saving, the carbon dioxide gas reducing and the pay back period. The result showed that Thailand had 902,000 litres of installed solar hot water system equivalent to electricity energy 6.573 GWh per year or 23.04 million baht per year, reduction of carbon dioxide gases 3.22 millionkgCO2 at the investment of 180.11 million Baht with average pay back period 7.87 year.