Abstract: The measurement of (n,2n) reaction cross section induced by fast neutron plays an important role in nuclear reaction mechanism research and nuclear technology applications. Experiments have been carried out over the last several decades. However, most experiments focused on solid samples including 89Y, 151Eu, 203Tl, 232Tl and so on, and there are few reported results about gaseous samples such as xenon and krypton. The 78Kr(n,2n)77Kr reaction cross section was poorly studied, and this work aimed to measure the cross section induced by 14.8 MeV neutron. The measurement was performed by the activation method, and the experiment was carried out at the Cockcroft Walton Accelerator of China Institute of Atomic Energy (CIAE). A cell made of PMMA was used as the gas container, and the 78Kr gas (enriched to 99.5% abundance) was injected into the cell using a syringe. The mass of 78Kr gas sample was determined by weighing method. Quasi monoenergetic neutron beams with a yield of about 3×1010 s-1 were produced via the T(d, n)4He reaction (Q=17.6 MeV). The ion beam current was about 250 μA, and the average energy of D+ particles was about 300 keV. The neutron flux determination was accomplished by attaching two 93Nb foils (same diameter as the cell and thickness of 50 μm) with purity of 99.99% to the upstream and downstream faces of the cell. The sample was placed at 0° angel relative to the incident D+ beam direction and centered about the TTi target at a distance of about 10 cm, where the neutron flux was 2×107 cm-2·s-1. After irradiation, the activities of 77Kr and 92Nbm were measured by offline gammaray spectrometric technique using a precalibrated HPGe detector on the surface of the detector end cap due to their low activity. To obtain detection efficiencies, another irradiation experiment was carried out individually to produce 77Kr and 92Nbm with high activities, which were used to measure the count rate ratio of the characteristic γray at a distance of 25 cm and on the surface. The efficiencies at the position of 25 cm were obtained by interpolating the efficiency curve which was pre-calibrated using various standard γ sources. Besides, Monte Carlo codes were written to calculate correction factors caused by geometry difference and neutron flux difference. As a result, the cross section was determined to be (464±19) mb, and was compared with the literature data and evaluations. The result of this work is much higher than that of previously reported. The result is helpful to the analysis and evaluation of 78Kr(n,2n)77Kr reaction cross section.