Reforestation has a great potential to fix carbon dioxide (CO2) in the atmosphere through photosynthesis of biomass and biomass energy conversion. The magnitude of CO2 fixation depends on the potential land area. We have proposed a potential area of about 350 Mha for reforestation in tropical and semi-tropical areas. If such land is used for energy plantation aimed at substitutive energy production by power generation, the CO2 mitigation effect can be evaluated on the assumption of short rotation energy plantation (6-year rotation), plantation area (1Mha), biomass productivity (10.5 dry ton/ha/y), energy efficiency of power generation (0.27), etc. Assuming these factors, the net carbon dioxide mitigation effect is 1.4 billion t-C on 340 Mha.