Petroleum, being dense liquid energy, dominates as world's transportation fuel, is said to be peaking out, then its production level will decline and the gap between demand and supply might expand year by year. In this paper, biofuel as a gap-filling transportation fuel, its potential energy volume, was investigated using various knowledge from papers, by unifying the knowledge into the same dry basis thermal energy terms. Agricultural land availability is limited. Therefore, the most important thing is to maximize land use efficiency (energy yield per hectare). It was found that sugarcane and sweet sorghum, which are suited for wet process like ethanol fermentation, and energy crops, either woods or grasses, which may be more suited for thermal process (gasification), are the most efficient feedstock in terms of biomass yield level. However, sugarcane and sweet sorghum converts only 40 % of its biomass into ethanol, while almost 100 % of energy crop biomass can be used for conversion. Therefore, it is concluded that the most efficient feedstock is energy crop either woods or grasses. Energy crops were also found to be more environmentally friendly with better water use efficiency (WUE), accompanied by C4 grasses, and better nitrogen/nutrients use efficiency (NUE), accompanied by C3 woods. In terms of land availability, 100 million ha land-scale is assumed to become available as least unit, which might be expanded up to several times. Energy crops, with their average yield of 15 t/ha, and their average energy content of some 18 GJ/t, would be resulted in some 27 EJ from 100 million ha land unit.