©2014 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd. 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 of79Se. The number of79Se atoms was determined from measured79Se/Se absolute ratios with the AMS system at the China Institute of Atomic Energy and the decay rate of79Se 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 an79Se sample which was cooled for many years to exclude the interference of short-lived nuclides, the extraction of SeO2-molecular ions, that results in a suppression of the79Br background by as much as about five orders of magnitude. Also, an AMS measurement of the absolute ratio of79Se/Se was developed to avoid systematic errors. The results show that79Se/Se is (2.35±0.12)×10-7in the reference sample and the radioactivity of79Se is (1.24±0.05) Bq/g, so the half-life of79Se is (2.78±0.18)×105a.