Abstract: Objective To compare the unconsciousness prediction probability(Pk) estimated by approximate entropy(ApEn) and bispectral index(BIS) during the sedation produced by target-controlled infusion(TCI) of propofol.Methods Twenty patients(ASA class Ⅰ-Ⅱ) scheduled for elective lower limb operations under epidural anesthesia were studied.TCI of propofol was initiated at target plasma concentration of 0.5 μg/ml,and increased by 0.3 to 0.5 μg/ml until loss of consciousness,and then decreased until consciousness recovery.Each target plasma concentration of propofol lasted for 12 min.ApEn and BIS were recorded simultaneously during the periods of consciousness and unconsciousness every 3 min.The Pk of unconsciousness with ApEn and BIS were calculated and compared.Results ApEn was 0.84±0.05 in the state of consciousness and 0.71±0.06 upon loss of consciousness,and BIS in the corresponding stages were 80.2±6.2 and 67.3±7.9,respectively.The Pk of unconsciousssness between ApEn(Pk=0.97±0.06) and BIS(Pk=0.91±0.11) did not significantly differ(P>0.05).Conclusion ApEn as well as BIS is acceptable for predicting consciousness and unconsciousness produced by TCI propofol.