The principal application of autoignition temperature (AIT) is to define the maximum acceptable surface temperature in a particular area, usually for electrical classification purpose. However, although AITs are indispensable information for safely handling flammable liquids, the reported AITs of flammable liquids in different data compilations are very much diverse. Sometimes, the difference in separate data compilations might up to more than 300 K. This article aims to explore the quantitative effects of flask material, ambient temperature, and ambient humidity on the accuracy for measuring AIT via the method of ASTM E659. To effectively analyze these factors simultaneously, the L-9(3(4)) orthogonal arrays are used to allocate experiments, and experiments are then carefully conducted in a temperature- and humidity-controlled laboratory chamber. It is found that ambient humidity does not affect the measured AIT of ethanol, but both flask material and ambient temperature are significant factors in measuring AIT of ethanol. An experiment of measuring AIT of ethanol conducted with a flask material of quartz is found to result in a higher AIT value than the one conducted with a flask material of borosilicate by 20 degrees C. A quadratic relation between the measured AIT of ethanol (y) and the ambient temperature (x) is found, and it is also found that a quadratic polynomial of y = 3.450 x 10(-2) x(2) - 1.454x + 3.711 x 10(2) could properly fit this relation with R-2 = 0.9939. According to aforementioned quadratic relation, the ambient temperature at which the lowest AIT of ethanol appears is about 21 degrees C.