Ｏｂｊｅｃｔｉｖｅ　To observe the effects of dexmedetomidine on postoperative cognitive function in elderly patients undergoing coronary artery bypass graft（CABG） surgery.　Methods　Sixty patients， aged 60 y-70 y， American Society of Anesthesiologists（ASA） class Ⅱ or Ⅲ， scheduled for CABG were randomly divided into 2 groups（n=30）. Group D received Dexmedetomidine 0.6 μg/kg intravenously over 15 min before induction of anesthesia， followed by infusion at 0.2 μg·kg-1·h-1 until the end of operation， while group C received the equal volume of normal saline. Jugular bulb blood were collected at 5 min before the beginning of operation （T1）， 30 min after the beginning of cardiopulmonary bypass（CPB） （T2）， 30 min after the end of CPB（T3）， 6 h after the end of surgery（T4）， and 24 h after the end of surgery （T5） to detect the serum levels of tumor necrosis factor（TNF）?蛳α， interleukin（IL）?蛳6 and S100β protein. The cognitive function was evaluated in the light of mini?蛳mental state examination （MMSE） 1 d before surgery， 1 d after surgery， 4 d after surgery and 7 d after surgery．　Results　There was no significant difference in the serum level of TNF-α， IL-6 and S100β between the two groups at T1. The serum levels of TNF-α， IL-6 and S100β in group D at T2-T5 were lower than those in group C （P＜0.05）. The serum levels of TNF-α， IL-6 and S100β were higher at T2-T5 compared with that at T1 in these two groups （P＜0.05）. The serum level of S100β increased obviously at T2 compared with T1， reached a peak at T3， and then gradually declined. There was no difference of MMSE value between the two groups before operation. The MMSE values in group D and in group C at 4， 7 d after surgery were significantly lower than that before anesthesia［group D （25.2±1.4） vs （26.6±1.3） vs （29.3±0.8）， group C （23.2±1.0） vs （25.6±1.3） vs （29.4±1.0）］ （P＜0.05）. The MMSE values in group D at 4， 7 d after operation were significantly higher than that in group C（P＜0.05）. The incidence rates of postoperative cognitive dysfunction（POCD） in group D were significantly lower at 4，7 d after operation than that in group C（36.7% vs 46.7%， 30% vs 36.7%） （P＜0.05）.　Conclusions　Dexmedetomidine could effectively decrease the incidence of postoperative cognitive dysfunction in the elderly patients undergoing CABG， which may be related to inhibiting inflammatory cytokine responses and S100β protein expression.