To reveal the dependence the flash point of flammable liquids have on altitude, a series of field tests at different altitudes in Tibet plateau were carried out by aid of a portable flash point measuring apparatus. The five tested altitudes are 3650, 3950, 4250, 4500, and 4750 m in Tibet plateau, as well as a benchmark altitude of 58 m in Hefei (a sea-level city). The test results show the flash point has a nonlinear dependence on altitude for all the tested flammable liquids. The subsequent theoretical analysis indicates that the reciprocal of flash point depends linearly on the logarithmic of altitude, which in general agrees with the test results especially at relatively low altitudes. The study indicates that the fire hazard of flammable liquids increases as the altitude increases, which is an important consideration for the safety design of aircraft fuel tank as well as the fire safety management of oil production and transport in high-altitude plateau. Copyright © 2013 John Wiley & Sons, Ltd.