The kinetics of pyrolysis and coke deposition of atmospheric gas oil (AGO for short) were studied at atmospheric pressure. The temperature, steam/AGO ratio, and residence time were varied in the range 1063-1208 K, 0.2 mol/mol, and 0.02-1.2 s, respectively. Mathematical models for pyrolysis and coke deposition have been proposed. Ethylene and aromatics are found to be the coke precursors. The study shows that the coking process in AGO pyrolysis is controlled by reaction kinetics.