This paper describes the basic principle and experimental arrangement of accelerator mass spectrometer based on 2.5 MV Van de Graaff accelerator

and the concentrations of tritium in the heavy water samples have been determined. For comparision the measurement was also carried out with radioactive method. The experimental result has demonstrated that tritium at concentration of 1 part in 10

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can be determined with a 1—2 MeV Van de Graaff accelerator mass spectrometer

and the potential sensitivity would be 2—3 orders higher. The prospect and problem on this small accelerator mass spectrometer have also been discussed.