Abstract: The classification of ignition sources and calculation of ignition probability was briefly summarized in this paper. Firstly, the classifications of ignition source were introduced, and the different results of immediate ignition and delayed ignition were shown using event tree method. Secondly, two approaches for the determination of ignition probability were introduced, i.e. ignition probability calculated with accident statistics and ignition probability based on the classification of ignition sources. The accident statistics of different processes were briefly summarized, including flammable material transportation, blowout, storage tank cracking, and long pipeline transportation. The ignition probabilities of different kinds of ignition sources given by TNO and HSE were also introduced. It's believed that the research on the ignition probabilities of different ignition sources in domestic petrochemical plants is very important.