Abstract: A method was set up by GC/MS-quantitative method with ISTD coffeine which can determine a new hyphosis drug-Melatonin. The Standard curve of Melatonin in blood is Y = 0. 0582 + 2. 6067X(r = 0. 9994). The blood concentration of Melatonin were determined in a patient after 20 mg Melatonin was administered, the Cmax was 140. 47 ng/mL at Tmax = 1h, then Melatonin distributed and eliminated rapidly in the patient,and then not accumulate in the patient's body.