The energy balance and CO2 mitigating effect of a liquid fuel production process from microalgae using thermochemical liquefaction were studied. Thermochemical liquefaction has the advantage of treating wet materials compared with direct combustion, gasification and pyrolysis, because it does not require a drying process. The yield of liquid fuel produced from Botryococcus braunii and its lower heating value were high compared with those of Dunaliella tertiolecta; therefore, the energy inputs for cultivation and separation of B. braunii were calculated to be smaller than those of D. tertiolecta. The energy input for fertilizers of B. braunii was also smaller than that of D. tertiolecta. Based on these differences, the liquefaction process using B. braunii was suggested to produce net renewable energy, but not that with D. tertiolecta. if a 100 MW thermal plant using coal would be replaced by liquid fuel produced from B. braunii, the quantity of CO2 mitigation could be 1.5 x 10(5) t year(-1) and 8.4 x 10(3) ha of microalgal cultivation area could be necessary.