Thailand has a high potential to utilize renewable energy for electricity generation especially from agricultural waste; however, at present only a small fraction of biomass is used for energy purposes. This study aims to estimate the potential of biomass power generation and its impact on power generation expansion planning as well as mitigating carbon dioxide emission from the power sector. The harvest area and crop yield per area are taken into consideration to estimate the future biomass availability. The supplies of biomass are then applied as a constraint in the least cost electricity generation expansion-planning model. The cost of CO2 emissions is also added to the fuel costs as carbon taxation to make biomass power generation competitive to fossil fuels, then the optimum value of CO2 charge is found out. In addition, levels of CO2 limitation from power generation are also introduced to mitigate CO2 emissions.