Abstract: In this paper,the method and results of three thermal modelling experiments on clay minerals are introduced; the one under normal pressure and varing heating temperature.the one with different heating time and the one under high pressure and varing heating temperature in alkaline mediums. Besides,the effects of temperature and sustained heating time on clay minerals,the relation of clay dehydration to hydrocarbon migration and the changes of clay minerals under high temperature and pressure are also discussed. The author considers that temperature and sustained time are the important conditions of mutual compensation during the diagenetic transition of clay minerals, and the pressure is a secondary factor. The conversion of montmorillonite to illite is advantageous to the increase of porosity and permeability.