Recent studies have shown that deep anesthesia is associated with poor outcome. There is still lack of randomized controlled trials with large sample size on the effect of depth of anesthesia on the postoperative outcomes in patients undergoing brain tumor resection. The investigators are performing a randomized and parallel group trial. The aim of the study is to determine whether there is a causal relationship between the depth of anesthesia and postoperative clinical outcome in patients undergoing supratentorial tumor surgery.

Inclusion Criteria:

• Patients scheduled to receive elective supratentorial tumor resection under general anesthesia from 2017 to 2019 will be recruited in the trial. Inclusion criteria include ages between 18 and 80 years old, American Society of Anesthesiologists (ASA) physical status Ⅲ-Ⅳ, surgery duration expected to be equal or more than three hours, postoperative hospital stay expected to be equal or more than five nights, and monitoring BIS throughout anesthesia.

Exclusion Criteria:

• Patients who undergo emergency or awake craniotomy surgery, or unable to present the written consent will be excluded from the trial. The patients whose incision site conflicts with the placement of BIS electrode on the frontal and temporal lobe will also be excluded from the study.