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The accelerator mass spectrometry (AMS) is an effective method for the determination of the half-life of long-lived radionuclides. In this paper, we report a method for measurement of the half-life of 79 Se. The number of 79 Se atoms was determined from measured 79 Se/Se absolute ratios with the AMS system at the China Institute of Atomic Energy and the decay rate of 79 Se was determined by counting the emitted β-rays with a liquid scintillation spectrometer. The major improvements of our measurements include using the high abundance of an 79 Se sample which was cooled for many years to exclude the interference of short-lived nuclides, the extraction of SeO − 2 molecular ions, that results in a suppression of the 79 Br background by as much as about five orders of magnitude. Also, an AMS measurement of the absolute ratio of 79 Se/Se was developed to avoid systematic errors. The results show that 79 Se/Se is (2.35±0.12)×10 −7 in the reference sample and the radioactivity of 79 Se is (1.24±0.05) Bq/g, so the half-life of 79 Se is (2.78±0.18)×10 5 a.