Objective:To explore the value of bispectral index (BIS)used in critically ill patients under sedation in intensive care unit (ICU).Methods:Thirty patients with mechanical ventilation in ICU were included in the study. Patients were sedated by midazolam or propofol according to the illness.The value of BIS and Ramsay score were monitored,and 2~4 points were the reasonable sedation for Ramsay score.The correlation between the value of BIS and Ramsay score,and the influencing factors were analyzed;meanwhile,heart rate (HR),respiration rate (RR),mean arterial pressure (MAP) and saturation of peripheral oxygen (SpO2) were monitored respectively. Results:The vital signs were more stable after sedation than before sedation.The value of HR, RR, MAP was decreased after sedation (P0.05). There was no significant difference in SpO2 before and after sedation (P0.05). The value of BIS significantly decreased with the deeper level of sedation, and correlation between the value of BIS and Ramsay score was significant (P0.01). Conclusions: The value of BIS is well correlated with Ramsay score in assessing the depth of sedation in patients with mechanical ventilation in ICU. The value of BIS can monitor the depth of sedation objectively and reduce adverse stimuli.