Abstract: The paper studied the method of determining trace thallium in biological samples. Rapid and complete dissolution of urine, serum, liver, kidney, brain and muscle was achieved with a mixture of nitric acid, perchloric acid and hydrogen peroxide. Optimal instrument operation parameters were selected. Element of Re was chosen as internal standard to compensate matrix effect. The detection limit was 0.008 ng/mL, the recoveries of spiking samples were from 93% to 110%, RSD are from 1.8% to 5.7 %. The study proved that this method was rapid, sensitive and accurate.