Abstract: The ignition processes of four types of magnesium powder dust layers at constant heated plate were investigated. In this respect, main concerns are temperature distribution inside magnesium powder layers at precritical, critical, and postcritical temperature of hot plate. The results showed that the ignition of magnesium dust layer would not occur at the maximum temperature of hot plate determined by standards GB/T 16430-1996 and IEC 61241-2-1-1994. The dust layer would come into a long-duration stable temperature after a short period of rising temperature. The critical temperature of hot plate and the ignition delay time increase with particle size. As the temperature of hot plate is larger than the critical one, the higher the temperature of hot plate is, the shorter the ignition delay time of dust layer is.