Abstract: The exact mass of poisonous rodenticide tetramine (C4H8N4O4S2, accurate mass 239. 9987) was determined by gas chromatography-time of flight mass spectrometry (GC-TOFMS) with home-made mass calibration method. The elemental composition of the molecular ion was determined to be C4H8N4O4S2 with mass error less than 2 mamu. The calibration curve of tetramine is linear (correlation coefficient: 0. 999) within the range of 0. 10 -10. 0 mg/L using n-C15H32 as internal standard. The limit of detection is 40 pg(S/N>5). The samples of a city in southern China were analyzed by liquid-liquid extraction and GC-TOFMS, and tetramine was detected in food and serum samples taken from site and poisoned victims. The results are used to evaluate the therapic effect of the poisoned persons.