Abstract: Objective To investigate the effects of preoperative administration of dexamethasone on serum levels of cortisol in patients undergoing gastrointestinal surgery. Methods Eighty patients scheduled for elective gastrointestinal surgery were included in this study, and they were randomly divided into control group (Group C) and dexamethasone group (Group D). Patients in Group C received 2 ml of normal saline 1-2 hour before surgery while patients in Group D received 10 mg of dexamethasone. Serum levels of cortisol were determined on the day before surgery (T0), post-operative day 1 (T1) and day 7 (T2). Results There were no significant differences in serum cortisol levels on T0 and T1 between the two groups (P> 0.05). Compared with Group C, Group D had significantly lower serum level of cortisol (345.8 nmol/L vs. 832.8 nmol/L, P< 0.05) on T1. Compared with T0, patients in group C had significantly higher levels of cortisol on T1(P< 0.05), while there were no differences between T0 and T1 in Group D (P> 0.05). Conclusion Preoperative administration of 10 mg of dexamethasone can decrease the serum levels of cortisol on postoperative day 1, and shrink the fluctuation of serum levels of cortisol.