Oil sludge, if unused, is one of the major industrial wastes requiring treatment from petroleum refinery plants or the petrochemical industry. It contains a large amount of combustibles with high heating values. The treatment of waste oil sludge by burning has certain benefits; however, it cannot provide the useful resource efficiently. On the other hand, the conversion of oil sludge to lower molecular weight organic compounds by pyrolysis not only solves the disposal problem but also has the appeal of resource utilization. The major sources of oil sludge include the oil storage tank sludge, the biological sludge, the dissolve air flotation (DAF) scum, the American Petroleum Institute (API) separator sludge and the chemical sludge. In this study, the oil sludge from the oil storage tank of a typical petroleum refinery plant located in northern Taiwan is used as the raw material of pyrolysis. Its heating value of dry basis and low heating value of wet basis are about 10681 kcal kg(-1) and 5870 kcal kg(-1), respectively. The removal of the moisture from oil sludge significantly increases its heating value. The pyrolysis of oil sludge is conducted by the use of nitrogen as the carrier gas in the temperature range of 380-1073 K and at various constant heating rates of 5.2, 12.8 and 21.8 K min(-1). The pyrolytic reaction is significant at 450-800 K and complex. For the sake of simplicity and engineering use, a one-reaction kinetic model is proposed for the pyrolysis of oil sludge, and is found to satisfactorily fit the experimental data. The activation energy, reaction order and frequency factor of the corresponding pyrolysis reaction in nitrogen for oil sludge are 78.22 kJ mol(-1), 2.92 and 9.48 x 10(5) min(-1), respectively. For precise use, the two- and three-reaction models are proposed to describe the pyrolysis results. Among the three models proposed, the three-reaction model gives the best fit. These results are very useful for the proper design of the pyrolysis system of the oil sludge under investigation.