The CO2 mitigation from the power sector in Thailand can be accomplished through both the technological substitutions in supply-side options, and the reduction of power generation through adoption of demand-side-management options. The traditional power generation expansion planning has focused only on supply-side options. However, in this study both supply-side and demand-side options are simultaneously considered in a long-term integrated resource planning model, to determine the least-cost options. Results of the analyses show that during 2003-2017 the use of efficient demand-side technologies and the efficient power generation technologies would reduce CO2 emission by 8.4% compared to the base case. When CO2 reduction targets are introduced, the installed capacity of and generation from clean supply-side technologies are found to increase. The long run average cost in the base case is found to increase from 3.10 to 3.22 US cents/kWh if the target of CO2 emission reduction by 30% is to be achieved. In addition, the sensitivity analyses are carried out to determine the effect of some parameters on the generation plan.