Using HTDMAB with different dosages as the surfactant, the authors modified montmorillonites and then described their free swelling properties in water and toluene on the basis of free swelling ratio experiments. A careful analysis led to the discovery that the free swelling ratio of the modified montmorillonite tends to decrease in water but increase in toluene, and that when the dosage of surfactant is around 15%, the swelling ratio of modified clay is rather low no matter whether it is in water or in toluene. This means that only when the de.age of the surfactant is strictly controlled can the modified montmorillonite without water and oil affinity be prepared. This conclusion is also applicable to the reservoir core rocks, and hence the damages caused by the clay minerals swelling in the water and the superfluous oil molecules adsorbed on the surface of the minerals can be prevented beforehand.