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 79Se. The number of 79Se atoms was determined from measured 79Se/Se absolute ratios with the AMS system at the China Institute of Atomic Energy and the decay rate of 79Se 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 79Se 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 the 79Br background by as much as about five orders of magnitude. Also, an AMS measurement of the absolute ratio of 79Se/Se was developed to avoid systematic errors. The results show that 79Se/Se is (2.35±0.12)×10-7 in the reference sample and the radioactivity of 79Se is (1.24±0.05) Bq/g, so the half-life of 79Se is (2.78±0.18)×105 a.