Abstract: The changes in clay mineral compositions and system fabrics in reservoir sandstone were studied under the high temperature－pressure and fluid circulation condition. The original samples contain quartz, feldspar and clay m inerals. Two core samples were taken for experiments at the temperatures of 150 ℃, 200℃, 250℃ and the pH values of 9, 11, 13. We find that the condition with a temperature of 250℃～300℃ and a pH of 9 is good for enhanced oil recovery ( EOR). The samples were also studied by means of X-ray diffraction and electron m icroprobe analysis. The results suggested that the transformation and migration of the clay minerals is the most important factor that causes damage of reservoir. Some tra nsitional products such as illite, zeolite, gibbsite and montmorillonite, were f ound. It's also detected that some ions in the injection water heavily infl uence EOR because they control the clay minerals reactions. How to prev ent the appearance of swellable and migratable clays is the key to protect perme ability of the reservoir. The results of this paper can be applied to the process of the steam injection or flood injection in reservoir sandstone.