There is a significant relation between postoperative cognitive dysfunction and increasing age, increasing duration of surgery. But, up to now, the effect of the depth of anesthesia on POCD remains controversial. This study is designed to investigate whether the depth of anesthesia affects the incidence of POCD, we will use propofol for total intravenous anesthesia in elderly patients whose depth of anesthesia was monitored and adjusted by bispectral index (BIS). The early and long-term incidence of POCD will be compared between deep and light-anesthesia groups.