The surface modification of minerals has been studied by infrared spectrum (IR)before. It is usually suggested that surface modification is completed if the organic group absorption, such as -CH2 or -CH3, appears in the IR spectrum of modified samples. Nevertheless, this method only tells us that the coupling agent exist in the samples, but fails to tell us whether the coupling agent molecules are really linked to the ions on the surface of the minerals or not. This paper tries to study the surface modification of fired kaolin by using MAS NMR on the basis of the following consideration: if the ions on the surface of minerals are linked with the coupling agent molecules, it can result in the change of such chemical environments around the ions as charge density, which can be detected by MAS NMR technique. The sample from the tonsteins in the coal-bearing strata where the kaolinite content is up to 90% is ground to 1250 mesh and fired in an oven at 950℃ for 12 hours. The fired kaolin is modified by silane coupling agent under a certain program.The comparative study on the 29Si and 27Al MAS NMR of modified and unmodified fired kaolin shows that the chemical shift of -106×10-6of 29Si is unchanged between modified and unmodified samples, but 27Al changes obviously. The chemical shift of 5.44×10-6 and 65.69×10-6 of 27Al in unmodified samples are separately shifted to 3.8×10-6 ～ 4.4×10-6 and54.6×10 -6 ～59.9×10-6 after modification. This result shows that the modification is completed by linking Al on the surface of fired kaolin with the silane coupling agent molecules.