Abstract: To assess corn and cassava fuel ethanol production energy efficiency, the authors estimate life cycle energy consumption of corn and cassava fuel ethanol, allocate energy uses between fuel ethanol and its byproducts, and finally, work out net energy value (NEV) and net renewable energy (NREV). NEV and NREV results of corn and cassava under changed yields and chemicals uses were given. The results show that NEVs of corn and cassava ethanol are 1.472 MJ/L and 2.417 MJ/L, respectively, and NREVs of corn and cassava ethanol are 1.474 MJ/L and 2.459 MJ/L, respectively. Corn and cassava ethanol utilizations as energy and renewable energy are feasible, and cassava ethanol has better feasibility than corn ethanol. Yield and chemicals use are key factors to improve NEV and NREV. Better byproducts utilization and substitution of organic fertilizer for chemical fertilizer help to improve energy efficiency and renewability.