Abstract: The natural cracking of crude oils in deep reservoirs has gained great interest due to continuously increasing depth of petroleum exploration and exploitation. Complex oil compositions and surroundings as well as complicated geological evolutions make oil cracking in nature much more complex than industrial pyrolysis. So far, numerous studies, focused on this topic, have made considerable progress although there still exist some drawbacks. However, a comprehensive review on crude oil cracking is yet to be conducted. This article systematically reviews the controlling factors of oil cracking from six aspects, namely, oil compositions, temperature and time, pressure, water, minerals and solid organic matter. We compare previous experimental and modelling results and present new field cases. In the following, we evaluate the prevailing estimation methods for the extent of oil cracking, and elucidate other factors that may interfere with the application of these estimation methods. This review will be helpful for further investigations of crude oil cracking and provides a guide for estimation of the cracking extent of crude oils.