Dexmedetomidine (DEX) induces a dose dependent sedation and has been widely used as an adjuvant sedative during regional anesthesia recently. We aimed to investigate the effective dose of intravenous single-dose DEX to induce consciousness inhibition in patients of different ages undergoing lower limb surgery with epidural anesthesia. Ninety-two patients were divided into three groups according to their ages. Patients aged 18-45 years, 46-64 years and 65-85 years in group Y, group M and group O, respectively. With the accomplishment of epidural anesthesia, a pre-calculated dose of DEX was infused for more than 10 minutes and the sedative state was assessed by Observer's Assessment of Alertness/Sedation (OAA/S) scale 30 minutes after the infusion. A modified Dixon's up-and-down method was applied to decided the dose of DEX for each sequential patient. The 50% effective dose (ED50) of DEX in the three groups were 0.40, 0.76 and 1.03 μg/kg, respectively. The 95% effective dose (ED95) in group O (0.54 μg/kg) was 45% of group Y (1.21μg/kg) and 64% of group M (0.84μg/kg). Besides, the incidence of bradycardia was more frequent with the increase of age. The present study indicated that the appropriate single-dose of DEX to induce consciousness inhibition should reduce with the increase of age in patients undering lower limb surgery with epidural anesthesia, especially in patients over 64 years old. This result may protect the old patients from excessive sedation and dose-dependent adverse reactions.