[Abstract] Objective: To investigate the effects of prophylactic magnesium sulphate on the sore throat after tracheal extubation with a double-lumen endobronchial tube （DLT）. Methods: Forty-eight ASA I orⅡpatients, aged 42–65 yr, scheduled for thoracic surgery with a DLT were randomly divided into 2 groups (n=24 each):control group (group C)and group magnesium sulphate (group M). Normal saline 2 ml or magnesium sulphate 500mg (2ml) were sprayed on the surface of the throat 30 s before tracheal induction in group C and group M respectively. Patients were assessed for incidence and severity of sore throat at 0, 2, 4, and 24 h after tracheal extubation . Results: Compared with group C, the incidence and severity of sore throat were lower at 0,2,4, and 24 h after tracheal extubation in group M. Conclusion: The prophylactic administration of magnesium sulphate is effective to reduce both incidence and severity of sore throat after tracheal extubation of a DLT.