Background: Nitrous oxide is a safe analgesics which has analgesic and sedative effects without loss of consciousness. The advantage of Nitrous oxide is that it has a rapid onset time and that we can modulate duration of its action. The purpose of this experiment is to obtain the safe and effective concentration of Nitrous oxide in Emergency Department as use of analgesics and sedative drugs. Methods: Twenty volunteers were investigated according to double blind method. Nitrous oxide was administrated to volunteer subjects at different concentrations; 33%, 50%, 67% each. Then we evaluated the Pain score, Blood pressure, Heart rate, Respiratory rate, Oxygen saturation and its side effects. Results: Pain Score were 8.4+-2.1 at 33%, 6.1+-1.5 at 50%, and 3.65+-2.2 at 67% of Nitrous oxide. Pain Score is decreased significantly following administration of Nitrous oxide at concentration of 50%, 67%(P<0.05). There was no difference between preadministration group and postadministration group. While no remarkable change in systolic blood pressure, heart rate, and oxygen saturation. Respiratory rate showed significant increase when 67% of Nitrous oxide was administrated. There was no side effect in administration of 33% of Nitrous oxide. When 50% of Nitrous oxide was administrated, there were dizziness 5(25%), paresthesia 2(10%), nausea 1(5%), palpitation 1(5%) and hyperethesia 1(5%). At 67% of Nitrous oxide, there were nausea 7(35%), dizziness 6(30%), paresthesia 5(25%), headache 5(25%), diplopia 2(10%), dysphoria 2(10%), salivation 1(5%), dyspnea 1(5%), nystagmus 1(5%), tinnitus 1(5%) and euphoria l(5%). Incidence of side effect increased as the concentration of Nitrous oxide was increased. Conclusion: We can conclude that safe and effective concentration of Nitrous oxide is 50% as analgesics and sedative drugs in the emergency department.