Abstract： Objective To measure the median effective concentration (EC50) of propofol for respiratory depression administered by target-controlled infusion (TCI) and evaluate the change of sedation. Methods Fifty ASA Ⅰ - Ⅱ patients undergoing elective surgery were divided into five groups (n=10) according to different plasma concentrations of propofol TCI: 1.5 mg/L (group Ⅰ ), 2.0 mg/L (group Ⅱ ), 2.5 mg/L (group Ⅲ ), 3.0 mg/L (group Ⅳ ) and 3.5 mg/L (group Ⅴ ). Respiratory function was monitored by side stream method (FiO2:1.0) with Datex-Ultima breathing mechanics monitoring device. The level of sedation was assessed by observers with the alertness/sedation scale (OAA/S) and the value of bispectral index (BIS). tided volume (VT), minute ventilation volume (MV), end tidal carbon dioxide tension (PETCO2), respiratory rate, SpO2, BIS, OAA/S, systolic blood pressure, diastolic blood pressure and heart rate were recorded before propofol TCI (T0), 1 min(T1), 3 min(T3), 5 min(T5), 7 min(T7) and 10 min(T10) after propofol TCI. EC50 of propofol for respiratory depression was determined using Probit analysis. Results VT decreased with propofol concentration inereasement in group Ⅱ , Ⅲ, Ⅳ and Ⅴ (P<0.05) more than that in group Ⅰ ( 1.2%-9.3% ), but RR and the duration of respiratory depression increased (P<0.05). Respiratory depression was found in one case of group Ⅱ , two cases of group Ⅲ, eight cases of group Ⅳ and ten cases of group Ⅴ. Apnea was found in eight cases of group Ⅳ and ten cases of group Ⅴ. The value of BIS and OAA/S decreased in all groups (P<0.05). The value of BIS was 60.2±4.0 for respiratory depression and 52.1±3.6 for apnea, and the score of OAA/S was 0. EC50 of propofol for respiratory depression was 2.65 mg/L(95% confidence interval was 2.41-2.90 mg/L). Conclusion Propofol TCI can realize satisfactory sedative and hypotic effect with mild respiratory depression at the plasma concentration of 2.0-2.5 mg/L.