Abstract: Hydrocarbon expulsion refers to the initial migration of oil and gas in source rocks. It is only after oil and gas being expelled from source rocks that the hydrocarbons can contribute to oil and gas mitigation and accumulation. The mechanism of hydrocarbons expelled from source rocks of high maturity has long been a controversial issue. In this study, the development of microstructures(micropore and microfracture)in original marlstone is examined with high- and post-maturity mudstone and limestone samples by means of environmental scanning electron microscopy, mercury porosimeter, porosity and permeability determination, and computed tomography scanning. Then, the source rocks are heated in an autoclave at constant temperature and pressure for 72 h of hydrocarbon generation and expulsion. In the thermal simulation experiment, the source rocks undergo a significant mineral recrystallization at the late diagenetic stage under the joint control of temperature and pressure. The mineral particles grow in size while substantial amounts of hydrocarbons are generated. A large number of fine particles are corroded by acid products, leading to the formation of micropores. The porosity and permeability of source rocks are significantly improved with no formation of new fractures. The results demonstrate that micropore is one of the primary channels for hydrocarbon expulsion from source rocks at late diagenetic stage.