The effects of pH and C/N ratio on bio-ethanol production using Saccharomyces cerevisiae were evaluated under anaerobic/aerobic condition. The growth of microorganisms was relatively lower under anaerobic condition (1.5 OD) than under aerobic condition (2 OD), whereas the ethanol production yield was higher as 0.45 g-ethanol/g-glucose under anaerobic condition than as 0.3 g-ethanol/g-glucose under aerobic condition. Weak acid (pH 4-5) was more productive to ethanol yield compared to that at neutral condition. Various C/N ratios from 2.5 to 30 were evaluated, and the ethanol yield was observed as 0.45 g-ethanol/g-glucose at 10 to 30 of C/N ratios. The yield at low C/N ratio (below 5) was less than 0.1 g-ethanol/g-glucose and corresponding glucose consumption rate was also less than 30%.